Chart Track are the "official" figures source for the UK. There are certainly a lot more 360s in the wild than PS3s in the UK, it's a pretty similar gaming landscape to the US here atm, however a short hop across the channel tells a very different story, one of total Xbox apathy (which is why the PS3 has now sold more units than the 360 overall in Europe).
GTA remains a very strong "PlayStation" franchise, so I'd expect more copies of GTA4 to sell per PS3 than per 360. I wouldn't read too much into it either way for a consoles supremacy in the marketplace. The Wii usually has half of the top 10 here
With the DS usually picking up the rest barring the odd big name PS3/360 release such as GT5:P and now GTA4.