As a Slovak I can confirm it's pretty easy to read both, although I hate the usage of cx, sx, ... instead of č, š, ... in Slovio.

I feel like Interslavic is closer in vocabulary to Slovak, but the form of the words is a bit foreign (reminds me of some other Slavic languages, although I can't quite figure out which one), while the opposite is the case with Slovio - the form of the words feels closer to Slovak, but the vocabulary itself feels more "Russian" to me.