Well... here's one unvarnished view, take it for what it's worth.
First of all, buying a Forza game blind is a bad idea. Some really love them, some can't stand them. I like a lot of things about F4, but I have issues racing the way I want to. Others don't, but you'll never know unless you try it. WITH a wheel, it's the only way to race in my opinion.
I'm sure you're aware of how awesome GT5 is or isn't, so I'll focus on F4.
It has amazing car dynamics. The way suspension rocks on all four suspension points, and collisions are just amazing.
Something is different about the 3D perspective vs other racers from GT to PC sims that doesn't suit me, and causes issues with me taking turns well. I haven't tried cockpit view too much, as last time I had problems with it too.
The cars are also VERY loud, even stock, and unless you drive in bumper view or have a good surround system, the tires can get easily drowned out until they squeal as you're pushing them very hard, which again makes taking turns troublesome for me.
Graphics are great. Cars look awesome, tracks are very well designed. Online is very good, but of course you have to pay for it monthly or by year.
The selling point for me is the mod and tuning system, VERY deep, LOTS of bodykit options for making race cars. And the crowning achievement, a Livery Painter which is extremely powerful. You'll need access to the Storefronts (pay for Live Gold) to get hold of a lot of decals and flags and stuff because T10 cheaped out on included elements for the Livery Editor. But with just a little work you can have a fetching looking ride.
Forza in a nutshell. Still, it's GT5 for me all the way. With a DFGT wheel on a stand, it feels about as good as a PC sim, which is just awesome. One fanboy's opinion. Merry Christmas!
