When one creates a conlang, it notionally has its own writing system (although in many cases actually working out the writing system is ignored, and everything is simply done in transliteration). If one is going to actually work out the writing system, how should one decide/evaluate whether to create ideograms, an abugida, an abjad, a syllabary, or an alphabet?
The answer mostly depends on your grammar and phonotactics:
Examples: Latin, Ogham, Mongolian. Alphabets are a pretty reasonable writing system for most spoken languages. I think there's a good reason why the majority of languages with standard written forms use some sort of alphabet. That said, they seem to take a long time to develop, so alphabets are unlikely for concultures that recently discovered writing.
Examples: Arabic, Hebrew, Tifinagh. Abjads are good for languages with few vowels and very few minimal pairs distinguished by vowels. It's especially good in languages with triconsonental roots or other forms that undergo dramatic vowel shift as part of regular grammatical processes to improve coherence across grammatical forms.
Examples: Devanagari, Ge'ez (Ethiopic), Canadian Aboriginal Syllabics. Abugdias are pretty good for languages with relatively simple syllable structure (ideally not more complex than (C)CV(C)). Based on the real world, it seems that they're best suited for languages with a high number of tones or phonations.
Examples: Hiragana*, Vai, Cherokee. Syllabaries are really only appropriate in languages with relatively simple phonology, ideally not more than (C)V (or adopt a pseudo-syllabary like the kana with separate symbols for coda consonants). Otherwise the number of characters grows exponentially, or will not do a very good job representing all segments of the language, though this is not necessarily a dealbreaker (cf. Cherokee).
* Hiragana is not a true Syllabary because each kana actually represents a mora, not a syllable.
Examples: Chinese, Cuneiform, Mayan. Logographs are best suited to isolating and analytic languages. The earliest writing systems seem to be logographies though, so cultures that only recently discovered writing are most likely to have logographies.
Trial and Error
Natural languages tend to end up with a writing system because it was, at some point in time in history, foisted on them. The Sumerians wrote in cuneiform. This later got foisted on the Akkadians, the Hittites, the Iranians, etc. The Romans wrote with an alphabet, and that got foisted on just about the whole world, even those prestigious languages that have long had their own systems. Mostly as a matter of convenience to online existence.
As a language inventor, you get to play with these forces and circumstances. And I at least advocate out several forms of writing for a language to see which you think is the better fit. Especially if you happen to be, as a matter of philosophy, a language inventor. If you happen to be a language discoverer, then you might come to the realisation that such-and-such a language uses a particular system, and you'll just live with it.
Trying out several systems at once, perhaps using a well known example text, will give you a good feel for how it meshes with the language structure. This exercise will also teach you how well (or how poorly) you understand either that kind of writing system or your own language's make-up! If you think a fixed syllable structure might benefit from a syllabary, then try it out! But if you discover that your "fixed syllable structure" is full of complications, exceptions, alterations, and so forth, you might find that a syllabary becomes unwieldy. That's not necessarily a bad thing --- scribes and writers have long had to deal with terribly complex writing systems (Egyptian! Sumerian! Chinese!)
Creating a few "temporary" writing systems for use with any later invented language project could also be useful. Doing this, you won't have to invent a whole new syllabary, a whole new alphabet for each future language project. The actual symbols won't necessarily be the symbols that get used for any language; they're just a tool to help you evaluate compatibility.
As for an evaluation tool, first I'd do some research into how many symbols various natural language systems have. In English, which uses an alphabet, we basically have 26 symbols. Abjads only notate consonants. So if English used an abjad, that would be 19 to 21 symbols (depending on how you count w & y). At the other end is Chinese, whose logogrammes number over a hundred thousand (according to WP's citation of a modern Chinese dictionary); but you'd only need about 2000 to read a newspaper.
This will you give a rough idea of how much work is going to go into devising characters for the invented language vs how much work is going to go into inventing the language.
Then, you'll want to consider how different writing systems work vs how your language works. No writing system perfectly captures the sounds of any language, except for the IPA. Everything else is a compromise. A language that is heavy on nuanced content words, a logography might be too unwieldy. A language with few lexical items might be very comfortable with logogrammes. A language with lots of vowels may not get along with anything but an alphabet: is PN abjad spelling for pan, pawn, pain, pean, pen, pin, pine, pane, pone, pon, or pun?
You'll also want to consider aesthetic qualities of the writing system. If you find cuneiform logogrammes are just the bees knees, then aesthesis might outweigh the difficulties of having to make hundreds of symbols. If calligraphy is more your style, then an alphabet or an abjad might be better choices.
Lastly, don't neglect combination systems! If you really like the aesthetics of logograpmmes, but don't want to bother with hundreds or thousands, consider a hybrid system where certain symbols have come to take on syllabic or alphabetic function, while certain words or classes of words continue to be written using full logogrammes.
I would say that depends on the structure of your language. If you have a small inventory of phonemes, then an alphabet might be best; if you have a fixed syllabic structure, then you might want to use that.
In general there is no reason why you only need one writing system. For toki pona you have a normal Latin writing system, but you also have ideograms and even a meso-American-style hieroglyphic writing system.
It depends what you want to use it for. Klingon has some very nice looking characters, but they are rather impractical, so mostly an ASCII transliteration is used. I would definitely recommend having a transliteration to make it easier to write the language on computers.
Other than that, it's up to you what works best!
The writing system of the language mostly depends on the grammar, syntax, and structure of the language. i.e. The writing system of a language always evolve in such a way that it would be easy enough to write the language without any difficulties in writing the language. To be more clear, here's the comparison,
|---------------------|----------------------| | Language | Writing | | type | type | |---------------------|----------------------| | Inflectional | Alphabet / Abjad | |---------------------|----------------------| | Agglutinative | Abugida / Syllabary | |---------------------|----------------------| | Analytic | Ideogram | |---------------------|----------------------|
As you can see in the above table, most of the world languages are by this. European languages and Semantic languages are highly inflectional, and they use Alphabet and Abjad respectively. Indian languages like Tamil(Dravidian), and Japanese(Japonic) are agglutinative, and use abugida and syllabary respectively. Chinese is a highly analytic language and it uses ideograms.
But it has to be noted that writing systems also depend on the languages and cultures that are nearby, but languages like Japanese still created its own script (Hiragana and Katakana, which are syllabaries) so that it would be easy to write.
Coming back to the question, choosing the right writing system is necessary as it is the first thing people are going to study in a language.
If you choose ideogram like Chinese, it would be easy if the conlang is analytical. It would be a nightmare to see Latin like highly inflectional languages in ideogramic symbols.
If your conlang is agglutinative, abugida and syllabary would be a great choice. Alphabets too make a fine choice, but trust me, representing agglutinating parts in alphabets is not efficient. Korean might be an exception.
Alphabets are universal, and could be used for any type of language. But if you want to increase the complexity of the language and need more elegant and unique language, it might not be great to choose alphabets.
Thank you for reading.