The PS3 has a more relaxed color pallet, but this gives everything a more realistic look. If you don't like the realistic look, and you have a HDMI connection, you can fiddle with your PS3 display settings and get it looking much more saturated. The textures are just as sharp as the 360 version, and there's little to no pop-in or screen tearing. The PS3 takes longer to load than the 360, but I took several different head counts in game at different locations, and I always found the PS3 to be more populated than the 360. This could be attributed to the dynamic nature of the world, but it's hard to tell. Either way, the difference was never more than five to seven pedestrians or three to four cars—it's not really much of an advantage.
The 360 is bright. Boy is it bright. Based on discussion history, this one will be claimed to look better, but it'll really be preference. The texture quality is the same on both systems, but the 360 has a more cartoon-like look. This isn't a bad thing. It just depends on preference. I'd compare the difference to that of Oblivion on each system. The 360 also suffers from some frame rate drops that are almost non-existent on the PS3. Installing the 360 version on your system's hard drive, however, will bring performance almost up to par with the PS3.
I know, you want details, but all I can confirm is split-screen on PS3 only. I was alone, so I didn't actually get to have any matches, but I could tell that the split-screen worked really well. There was a significant decrease in the amount of traffic and pedestrians, but it didn't ruin anything. This is understandable. Rendering two instances of Liberty City is a chore for any machine. I can't conclude one way or the other why the split-screen mode was not available on 360, but I assume by the drop in on-screen objects on the PS3 that the 360 just couldn't handle it. If you own a 360, don't worry. You have Xbox Live…so you're set.
*** AUTHORS NOTE ***
NOTE: Geez...Posted just 6 hours ago, and I've already got hate mail! Let me clarify a few things. First, the build I previewed was dated 2/21/08. I finished the entire game (and have been playing it since). Second, the 360's issues appear to be bugs, hence the disclaimer below. Third, I'm not a PS3 fanboy. The PS3 doesn't have any good titles (except for Uncharted), so how can someone be a fanboy? I'm purposefully harder on the 360 version because I've been called a 360 fanboy in the past. There's no winning here! Fourth, I've added a counter. When/If this page can rack up 500,000 hits, I'll post more detail in a *SPOILERS* section to further legitimize it.