Edwin may have a point there. Some other games look better on PS3, but with Rockstar the 360 versions seem to be superior.
The major factor that Rockstar is undoubtedly dealing with, here, is the fact that TVs only accept a finite number of input-resolutions. The most typical are: 480i/p, 720p, and 1080i/p. What some games do (Halo 3, CoD4, GTA IV on PS3, and, apparently, this) is take a lower-rendered resolution at 640p, and scale it up to a resolution that it can then send on to your TV.
So, as far as the TV knows, it's getting 720p of picture, but those pixels actually just consist of a lower resolution image stretched to fill that range. This either happens because rendering at a higher resolution takes too much space, or too much time (processing).
For the PS3, the issue is usually memory. Due to the system architecture, it only has 256MB of main memory, and something like 20-30MB of that is taken up by the OS (at least). So they're left trying to fit a massive game like Red Dead Redemption into something like 200MB of memory. The 360 is a bit more flexible with how its memory can be used (it has 512 MB that can be split in any fashion between the graphics card and the main CPU), so it's easier for developers to get the space they need.
An interesting thing about the PS3 is that it doesn't have a dedicated chip to handle just that scaling. The graphics card is capable of doing it, but unlike the 360, which can use its chip to scale to a bunch of different resolutions at no cost to the graphics card, the PS3 doesn't have that capability (which is why we still have people who complain that the PS3 can't output to 1080i).