I have both NFS Most Wanted 2012 and Forza Horizon.
Forza Horizon has absolutely stunning graphics, a good soundtrack, more single player content than NFS, more car selection and customization. The racing is also far more realistic, and the Forza devs and community release cool DLC content for free. However, Multiplayer got boring quite quickly for me. World also felt kind of empty.
NFS has addictive gameplay which is easier to learn, pretty good soundtrack, good graphics, it's really annoying when you crash mind. Multiplayer is excellent and you'll be playing it for hours, it has many game modes and it's just so much fun. However, short singleplayer and police cars can get annoying if you just wanna cruise (there should be an option to turn it off when not racing). Hardly any customization features and not much of a storyline. Not as realistic as Forza but fun nevertheless.
I honestly couldn't decide between them, just go with what your friends have, and if none of your friends have it then look at videos on YouTube and see which one would be more preferable to you.