I have played both versions of GTAIV and the best version is...

[Slimebeast]

I had a dream last night where I was playing both versions of GTA4 on two identical TV-sets (42 inch plasmas I think... the dream was a bit blurry, but I think they were Panasonics). I made a side-by-side comparison, and I'd have to say the X360 version was slightly better.