Forza 5 is the simulator, Forza Horizon is the half-arcade, half-sim version. 
Horizon is also the full terribad version. Games brutal. I think I'm getting old hahaha.
I like Horizon. 75 events entered, 75 1st place finishes... 
I like it too, 40+ events entered, 40+ 1st, but I got 2nd in one race. Apparently, an intake/exh/suspension/brake/tire FD with a 6.2 = drift missile. I couldnt get it for anything, and had race tires on in a rally cross. After 5 retrys I gave up and took my 2nd place.
I think its hilarious about the 6.2l V8 being swappable into anything. Can anyone say 1993 Legacy Sedan, complely stock minus 6.2? Its actually fun...best way to get skill points? blown 6.2 V8 Jeep, RWD Conversion, tires, brakes. Then, just drift up and down the longest highway in the game. Farmed like 15 skill points in like 30 mins ha!
That game is fun because you can just pick it up and play. When I want a more realistic experience, Ill just go grab GT5 on my PS3.