The AI in forza while aggressive and stick the fastest line at all costs at least provide a good race. GT was a giant time trial with cars on track.
Well..... maybe I'll add my 128 bits to this discussion.
I've just gone back into Forza 4 after essentially dumping it seven weeks after I got it, and the 360 has been collecting dust and cobwebs ever since, as I just don't care for the Box or its game library. I decided to give it another chance after racing in GT5 pretty much non-stop for almost a year, along with a couple of my PC sims, GTR and Live For Speed. While I remembered most of what made those games good - yes, even Forza - some of it got lost in the fog of memory. It was good to get reacquainted and re-learn some of the shortcomings of GT5.
I made a post about it some weeks ago, but I'm still a little yucky, and frazzled after racing in a certain game. In essence, all of them have very good physics. They give you a good race, with a caveat I'll get to presently. To one extent or another they put you in the car and on the track, and racing is pretty darn challenging. But Forza 4 has had the might and talent of more than 400 people working on it, including a Hollywood graphic studio to tune up the graphic engine, and they tore it completely open and reworked just about everything. As a result, Forza 4 could be the best racer on the market, with the possible exception of rFactor 2 which I haven't experienced. The car behavior is outstanding, the modding and livery painting capacity is world class, and the sounds will roar out of your speakers. Every time I roast tires in F4, I swear I can smell burned rubber! It really is that uncanny.
The upshot of my racing day with those other games is that it reinforced the impression I had of GT5 in that the racing is very "safe." Every car is well planted on the road as if it was race tuned, and car dynamics in a race are way too tame, even given that most tracks in almost every racer I've ever played are very smooth. I've been doing Arcade Mode races in GT5 almost exclusively, and cycling through literally hundreds of my Standard cars, and while the bots are fun to chase, they are pretty generic and mostly polite in their behavior, and tend to wimp out when you get any substantial lead on them. In contrast, Forza 4 feels downright dangerous. Taking a Golf GTi around Hockenheim was such a challenge that I had to practice the track for about 40 minutes before I felt remotely comfortable with racing. Throwing a sports car on Sports tires around those curves was a struggle. The car is a handful and it's a little scary trying to push it into limits that aren't safe to drive at. This hazardous aspect, the raw ragged edge of performance, is something GT5 lacks.
However... racing was painful. I hated it. The bots are rowdy brats and aren't shy about using you as an auxilliary brake or trying to bash you off the track. Unlike my car, theirs could make the usual Forza magic maneuvers, flying past me to make olympic late braking maneuvers near turns, fly around those turns while I struggled, and could whip their cars left and right without a problem, in a way that would have me skidding off the track if I tried them. And the stupid exhaust roar of every car from a VW Beetle on up drowns out the tire sounds, which I need badly to judge how much I can push a car around a turn. And your car, not the bots. In fact, it's truly annoying that I can be half a mile from the other cars, and their muted whines will STILL sound as if nearby, and STILL be louder than everything but tire smoking skids from me. Gah. And yes, I adjusted the sound balance.
After finally bashing and cheating my way to a victory, I grumpily put my G25 and 360 Leet stuff back in the basement and ran back to GT5 for the remainder of the night, as messing with Forza had eaten up just about all of the day. And the cars felt too safe, and I missed the sense of danger and that tire squeal, and the next day I had the crazy urge to race in Forza again. I didn't, still raced in GT5, but I peeked at the Forza boards and ogled those liveries everyone was making, and sighed. Monday I got the crazy notion to try Forza one more time. And I had a couple of fluke races that were fairly easy, and pretty darn fun. Plus, the Livery Editor thread had sprung to life again, and I thought maybe I'd try making a new racer to illustrate what I wanted to see in GT6.
Well... Forza turned on me like my drug casualty brother. Racing was utter hell again. I hated it more than ever. I just got done with a race on LeMans which had me spitting and cursing, and I resorted to turning the bots down to medium difficulty just so I could get away from the mother... in-laws. But that was only half a relief, because even with normal steering, tuned brakes, traction control and the brake guideline on, the car would start skidding if I so much as looked at it funny, and nearly every turn had me fighting to keep it on the track. AARGH.
There are still a lot of things I think are excellent about Forza, things every racing developer could stand to copy. And if I could, I'd transmogrify GT and Forza together to create a true Gran Forizmo. It would be the ultimate racer, and I wouldn't play any other racer ever, except the sequel.
Sadly, that's not going to happen. Right now, I'm letting the bot race while I catch my breath and calm down, and farm races for badly needed credits so I can buy more cars I may never want to race. It's sad, but I may be done with Forza, because if I can't push a car around a turn with any assurance, there's not much point to racing. And I wouldn't last a second online, racing like a shaky little kid and bashing into people.
So... yeah, GT6... guess we'll see how awesome sauce it is or isn't in July.