Same models, same images, same textures, same edge detection, same shadowing algorithms...
The system was rebuilt from the ground up, presumably to facilitate both systems. Different hardware, okay, but I'm not sure how much stuff they'll actually tweak considering this was made for both systems from the start (and okay, I'm presuming here too, but given the release date, I don't think it's a bad assumption to make). I wouldn't be surprised if the graphical differences end up being slight at best, and there's so much conflicting stuff in that thread, I'm having trouble taking anything seriously in it. I'd get together with a friend with a PS3 and a good TV, and do some HDMI 720p comparisons for yourself - then you'll know for sure.
Of course, if you're saying you don't have a friend with a PS3 well... you're with the many others not in Japan who want the PS3 version but don't have a PS3. ^^;
I'll probably run those, although I can confirm that it isn't a fully rebuilt system - it was mostly porting across the board. There is a PC build of im@s 2 that does exist, sort of.
I'll do some benchmarking myself - just be warned you can't actually DO a HDMI 720p comparison - the PS3's HDCP is complete - no ability to do trickery like the Xbox 360's implementation. (And if there's a HDCP breaker, I want to know, because I'm looking for one)
It'd have to be via component, and there's actually no technical difference, apart from possible interference on the cable itself.
Anyway, for most part, the PS3 version is what most people outside Japan will be able to get with only reasonable difficulty, so what you see is the only thing you're getting.
I will note that the X360's release isn't dead yet for a few reasons. For one, you don't get 8 catalogs for free. You only get the costumes free for the first 3, and the other five are interwoven in the following SIX catalogs. (First being reserved for the PS3 release song)
The other 5 come later on the PS3, so in actual fact the X360 players have gotten a six month headstart. Considering how Kyun! Vampire girl got, and some requests for Overmaster, well... we'll be waiting a while.