Saw this which compares same or similar locations of both racing games. Both got pros and cons but neither outdoes each other by a large margin IMO.
Pick your flavor and discuss technical differences.
http://www.gamereactor.se/nyheter/25153/Gran+Turismo+5+vs+Forza+3/
I wouldn't bother until proper comparison shots show up. I can't believe they haven't bothered to pick the same car / track combos properly, as there are plenty of cars and tracks that appear in both. That said, if we look at Laguna in particular, the track looks a bit better in Forza. (These are weird captures though - the powerlines never look like this when playing, not in either game I think)
As I posted on GAF:
These games have a lot of same track and car combos. You can even change the cockpit view of GT5 to match the one in Forza almost exactly.
Can't you do better? I would:
- pick a matching car/track combo
- set the speed measurements similar
- drive the cars at the exact same speeds (a shot at 0, 50, 100, 150km/h)
- do this from the different camera angles
- do a test for the AI cars separately. Harder to do slightly, but should still be possible (you can set cars to 'one make' for both)
Additionally of course you can also choose to try and find some of the best and worst looking bits for each game and compare those.
It's still a limited comparison, what with one game having to accomodate 50-100% more cars and such, having day night cycles and weather on some tracks, etc, but it's something at least.
Certainly though one thing for me is sure - apart from that the cars in Forza almost always look quite a bit worse (a few of the later Forza DLC cars I recently bought excepted, which look pretty good), and some of the tracks in Forza looking a bit better, the single player portion of Forza is tiny and boring compared to GT5s.