Coming in to post my opinions.
I am a Forza fan now, through and through. I never hated it, but I nevr thought much of it. I was a GT-addict. I even got my parents to pre-order the signature edition (I have the Apex book in my backpack as I type this).
But GT5 is a dissappointment. Not even my DFGT can save it. The game is heavily unfinished. The car list is filled with useless duplicates and missing some great cars. And I'm not even going into the Standard/Premium idiocy. Let's not forget the 17 RMs, barely any events, no livery editor, or much more.
Eventually, I was sick of it. I returned the game for a meager sum of money. A year later, I got a 360 and Forza 4.
Now, Forza 4 is incredible. It blew me away. When I first got GT5, I played it for 6+ hours straight. But only on that first day. It just couldn't hold my interest.
But Forza 4.....now, this was amazing. When I first got it on Christmas break, I spent at least 8 hours
each day. For at least 5 straight days. And even today, whenever I can, Forza 4 keeps me hooked to the Tv for hours.
Why? It's the sheer amount of things you can do. Hey, lets take that car I never used before for a race, and see what it does. Oh, I have no cars I want to take out today? No prob, lets head on down to the dealership and pick out a new one....hey, that one looks nice.
Hey, what would this car be like with a few mods? Let's see how it does upgraded. Man, it's kinda boring now. Let's head down to the storefront and buy a design for it...
See, that's what Forza 4 does better than GT5. There's so MUCH to do. Every day of turning on Forza 4 has new cars, new tracks, new things to do. New rivals, new liveries, new tunes, new crazy stuff like making a 700HP Pinto. Why? Because I can, dammit.
Also, I have to say that Forza 4 handles better than GT5. I don't drive in real-life (don't have a license), but Forza 4 seems to handle much more smoothly and better. GT5 was hard as hell on a pad, but I race in F4 with no aids at all (I even use manual with clutch and Sim steering) and it just controls better. But, it's not easier--my 599 GTO is still a complete handful to drive. But I can control it with enough skill, something that was much more difficult with GT5 and my RM Corvette.
In terms of graphics, Forza wins this one. I remember playing it for the first time and being amazed at how smooth everything looked. I had no clue why, until I realized: it's a rock-solid 60FPS. Not only does Forza 4 nearly look as good as GT5, its far smoother too.
In addition, every car in Forza has cockpits. I can vouch that the C32 AMG in game looks almost EXACTLY as my dad's own C230 on the inside, besides a few model-specific differences. Whereas in GT5, 20% of the cars have cockpits. Everything else doesn't have real ones. Also, F4's models, in my eye (and I have a good eye for these things), look 98% as good as GT5's premiums anyway.
Also, Forza lets me download/make
Itasha. That is AWESOME. GT5 doesn't. That isn't awesome. My Black Rock Shooter Camaro is really cool looking.