I have a TON of fun driving around in Horizon with my racing wheel(wheel $11.98 Target clearance)!!!!! At first, I was pissed because of the 'Better with Kinect' thing and it has no 'real' Kinect features, like Forza 4 does, other than voice. Got the wheel and it changed my mind! Good music! That game is HUGE! It counts the number of roads you drive, too. It has over 200.
Just got the Game Of The Year version of Forza 4 from Best Buy for $20. Haven't tried it, yet.
From the vids, Forza 4 if you want to race, Horizon if you want to drive around, freely, and go weeeeeeee!!!!!!!! I swear I get lost in that game.....mentally and literally!