Esperanto took its vocabulary from different European language families, especially Romance and Germanic languages. Beside that it ensured the word choice did not lead to ambiguities like homonyms and plural meanings. (Slovio will not need this.)
Also there is a strong word creation mechanism in Esperanto. You will need to know a fraction of the word stems in comparison to other languages, and be able to actively create new words, that you know are correct. A child, a mentally handicapped, an aged person will all be able to be fluent in the language.
Slovio unites the Slavic languages, with appropriate simplifications. It has the advantage to be "immediately" readable by users of the Slavic family. One may compare this language with Interlingua for Romance languages. But Slavic languages seem to be even more similar.
Concrete Esperanto I consider a more powerful language, but with its claim on more heterogeneous language origins, it has coincidental choices (Tuesday = mardo, month = monato), and a hard phonetical spelling (peace = paco [pátsoh]).
Also Esperanto has its own accented letters for phoneticism: ĉĝĥĵŝŭ (she = ŝi).
And Slovio is a more acceptable, familiar language, though non-Slavic loan words risk getting purged. Though not to the degree as in Interlingua.
Myself I am Esperanto user, and merely have read a bit on Slovio. It seems there are some small reforms (?) on Slovio, say on verbs. But that is less important (than with Esperanto). As Slavic languages can have their difficult parts, which in Slovio are simplified, I would be interested how Slovio would compare to Basic English, an English without unneeded, superfluous terms, and inflections.
Slovio is based on the ideas of Esperanto. The alphabet for instance, including the x-system. Suffixes, prefixes. It is also simplified.
Remains the "ideology:"
There is the (contestable) fact that when one first learns Esperanto and then French, one learns French faster, than immediately learning French.
I see this also for Slovio: it might make learning faster and more pleasurable, so when after that learning a more complicated Slavic language, one progresses better. Not sure whether faster.
A Serbian and a Bulgarian will be able to communicate, even without Slovio. Slovio as common lingua franca? Who knows. With respect to Russian it might have a harder political stand.
It might be a more homely language.