I've been a GT fan since first and have GT4 with Logitech DFP, the DFP alone makes GT4 twice as good. However, I am playing Forza right now because I wanted something new, and better looking. I don't have a HDTV tv, but I can hook up my xbox to my 2001fp Dell for 480p res. So I can't compare GT4's 1080i res to this. Oh well.
Anyways, Forza does lack something, the feeling of speed. Capped at 30fps, I've had several non-racing gamer friends look at me playing both GT4 and Forza and have them critique the feel of speed and they all agree Forza just doesn't feel like the car is moving fast. My other complaint is the lack of USB on xbox which limits me from using my beloved DFP. I don't want to buy another steering wheel, but I might.
Anyways, I think Forza and GT4 are pretty competitive with each other, but online play, damage, and limited customization makes Forza better. I'm currently 8% career mode, and 70% arcade mode done. Playing about an hour a day or so. Look for Xentar on xbox live if you wanna race.