Okay so since I haven't played Forza 4, Are the physics the defining reason that some pick GT5 over FM4? As I see it, Forza has more cars, customization, tracks (better quality tracks such as drag strips), and more people playing it but I have heard that its the driving itself that hinders its "perfection". Without flaming me please. I just am curious if it's worth going out and buying.
As others have said; physics-wise, I actually prefer Forza, not GT, and jumping back and forth between them more lately thanks to GT Academy has really put into focus why that is. Forza doesn't have more cars, but it arguably has the better list for your average enthusiast, especially if you're willing to buy the DLC packs. I purchased a Season Pass with no real experience with the series before FM4 (only about two weeks of FM3, as it came with my system), based on the strength of the FM3 DLC car lists, and I've been very happy with the packs. I'll also be picking up the Porsche pack as soon as the discount VIP version is available.
Customization definitely goes to Forza, especially with engine swaps and the obvious big draw: livery editor. Forza is less of a Pokemon-style gotta-catch-em-all game: all cars are available off the bat if you have the cash, and you can still test drive any of them in Arcade Mode. Technically, you only ever need one of any model of car since you can load numerous setups and paint jobs as you need them. No hunting for a desired car for weeks. The community features are also better: leaderboards on every track, for every class. A real auction house. Storefronts. I'm sure you've read
Tracks are a matter of opinion: the Nurburgring
is better in GT5, more realistic and accurate. But taking it and Spa out of the equation, I prefer the rest of FM4's track list, with Road America, Road Atlanta, and Infineon being my three most-driven tracks. GT5 does have the benefit of track DLC, though.
The only thing off with your post is that no, Forza doesn't have more people playing it. Sales aren't even half of GT5's, and some of the most-used leaderboards don't have the sort of numbers something like GT Academy has brought in (though that isn't an entirely accurate comparison, since GTA doesn't require GT5). Oh, and if you're so-inclined, GT's photomode is better. That's a source of frustration for me
.
Sorry for the long-winded reply (especially as it's somewhat off-topic), but hope that helped.