As a Pole, both languages are readable, but InterSlavic takes less effort and feels more natural. This FAQ matches my intuitions.

That said, I would be curious how it works for other Slavic languages. Or, in other words, is it sort of equidistant or favors one language or the other?

I mean, the FAQ claims that it is the easiest for Czechs and Slovaks (is it because of their geographical centrality and possibility that these languages are the closest to the "Slavic average"?) and harder for the South Slavs. However, and always, I would love to see any data.