Will games look better on 360 or PS3?

I was thinking of getting a PS3, and I know that multiplatform games like Assasin's Creed, GTA 4, DMC 4, and many others for both 360 and PS3 come. I like 360 ones because of the whole achievements thing, but who knows, it might look better on PS3. I know that Blu-Ray gives TONS of space, but will it make a HUGE difference? I mean achievements won't matter THAT MUCH, but if it's multiplayer, I know XBL is strong, how strong is the PSN?

All that matters really is graphics. I know if MGS 4 ever comes to 360, it'll be better on PS3 because of more space. But which one is better graphically on multi-platform games? I don't want some old gamespot comparison, just something from companies who make the best games like DMC 4.

PS3, but I would wait if i was you..