The issue is that what you get depends not just on the console, but the code.
And its here were it gets messy as of course you have code written just for the PS3 (Uncharted or R&C say) and code written just for 360 (with maybe a wee glance at the PC) like GoW and Mass Effect. Then you get code (allegedly) written for both using custom developed tools (normally on a PC) like DMC4.
Then you get code written for the 360 and ported...
The upshot is that of course not only will the code be of varying quality anyway, but that by the nature of the two consoles to get the best from both you need code really tuned to each (either via exclusives or efforts like COD4 and DMC4).
In this case (GTA) I'd say that if you squint and read between the comments from Rockstar (I'm ignoring all others as not being valid) then the following where the challenges they faced during development of the game:
1) PS3 architecture (and lack of familiarity with thereof)
2) 360 lack of HDD
As a result it seems the had 360 code ready earliest but it took the longest to optimize and they had to focus purely on streaming approaches.
The PS3 code came later and they were able to use HDD.
The result, from what I can see, is that they decided to drop resolution slightly on PS3 and increase post-processing effects and lighting. The result, coupled with the HDD, is a 'slightly' better looking game on PS3.
If you compare the actual specs of the two machines I'd say that graphicaly they should be able (in ideal circumstances - i.e. good well written and optimized code for each) be able to produce very similar results.
If the PS3 has a real advantage its more in the field of processing code (say AI or physics) as it has more raw processing horsepower available - but again, to see that in use someone's going to have to code for it. And with multi-platform games the trend is to get them the same not have one blow the other out the water (You can be sure EA didn't want to produce crap ports for PS3 initially for example).
What difference there is will only, I believe, truly be seen in exclusive titles that optimize every line for that console. The GoW2 is sure to become a benchmark for 360, while the next Uncharted, and GT5 are sure to show PS3 in a very good light also.
Try to be reasonable... its easier than you think...
