Ok, Forza Motorsport is designed to be a simulation. You can customize difficulty with things like flashbacks, assisted steering, assisted braking, ABS, traction control etc. You can also go as far as tuning your car, from tire pressures, to shocks and absorbers, even gear ratios etc.
Forza Horizon is the arcade version of Forza Motosport. Though it's more arcade like and damage can be set to visuals only. It still pertains the authenticity of all the cars and details. The tuning system has been brought across to Horizon 2 from Forza 5, but you don't have to fiddle with any of it. Think of Horizon as an upper-class version of NFS.
Here's a video of Gamestop playing the pre-release version. They give you a good inside look.




