From my experience in the demo, GT is clearly the best solution for serious, accessible online racing. I've almost never had any bad race, and when my SR and race pace got higher, I began racing with real, fast gentlemen.
It's just sad that the physics are still so basic. Just wish a game like AC or PC2 could spend more effort to introduce a similar matchmaking with at least equal penalty system.
The graphics of GT are clearly superior, but I couldn't really care less, racing with AC graphics is good enough for me.
About Forza, I was really interested in it until it came out and saw the gameplay and the online content. The physics look like NFS to me (I'd say +90% of youtube videos are about people drifting with controllers, and it looks like it requires no effort, once in a slide, the car will almost never over-oversteer, any acceleration will help stay on the drift rail like NFS unlike other games when it's the opposite effect and it's better to more often let go off the throttle and control the throttle very carefully to have a clean, nice drifting) and the online is really destruction derby.
It's true GT lacks content, but to be honest, it has plenty of GT3 and other race cars to race with, and actually VGT are mostly in another category which is a welcome addition. However, the true let-down, especially compared to GT6, are the tracks, and dynamic time and weather.