The big difference are price, quality and size.

The OneStep+ is going to be the largest size (same shape as the instax square but larger)

The OS will have more control if you use the app.. but the SQ6 has more control on the camera itself. I found most of the OS+ features to be relatively useless for most.. but it's nice to have. Without the app, I found the OS to perform much poorer (auto mode) than the 6.

The instax photos will be much more clear and naturally colored while the OS films will come with a much more vintage/retro feel with muted and tinted colors throughout.

Price is another area where you'll find that 8 polaroid photos will run roughly the same cost as 20 Instax square photos. Camera wise, expect to spend about $100-130 on a OneStep while the 6 can be found as cheap as $60 new with film.

I tried several OS cameras and wasn't a big fan. I feel the money is better spent on something like an sx70. On the instax side, I would also recommend looking at the Mini cameras if you're not opposed to the smaller size. Film is even cheaper and some cameras like the Mini90 have the most features of any instant camera