I haz both NFS and Forza H.
Forza beats it pants down with pretty much everything, unless you're really into arcade gaming...
As for cars on Forza, the packs are rolling out like mad, I'm pretty sure the missing cars above will be available at some point.
I did find some of the races to be very repetitive, especially the short courses...but still looooving it...
PS: get the speedwheel bundle for the game, it's pretty damn awesome (despite looking somewhat odd)