I prefer the Lomo Instant cuz there's more to explore, say for example, both got multiple exposure but s90 only got 2 exposure but lomo instant can literally multiple. the s90 if i'm not mistaken has only a 5sec long exposure but the lomo instant as long as you want. and really so much more to explore.

in short, what i always tell my friends, if you want something that is safe most of the time (auto mode), get the s90. if you want to explore and really get into experimenting different possibilities, get the lomo instant, but be ready to waste many shots. but it'll be worth it. and the picture turn out nicer in my opinion too, with that wide angle.