Interslavic seems to be south-slavic biased.
Purely subjectively, Slovio text reads more aesthetically pleasing. I think this might be because the south-slavic roots are more prominent in our east dialects spoken in the country side, while Slovio seems closer to me to the standard codified language.
I'm a native English speaker in the Czech Republic. I've been struggling to learn Czech for 8 years now. It is my 6th language, not that I'm very fluent in any of the others, but by far the hardest.
I find Slovio much easier to read and understand than Interslavic, which with my A2-to-B1 level Czech remains quite impenetrable.
I've travelled around 5 or 6 Slavic-speaking countries now and often show people Slovio, and though they've never heard of it, they all find that they can read and understand it without problem. I find that an interesting data point.