forza 4 owns gt5, you really need to invest in a proper expensive steering wheel like my friend has to get the most out of gt5, and if you do the game is simply sublime, with the controller the game is literally pants
gt5's graphics are marginally better, physics are pretty much non existant on the controller, with the steering wheel the physics are pretty much as good as your gonna get
ive not played forza with a wheel but the controller works well with this game, the game is more aimed at being fun rather than gt5's full simulation, i feel forza has a nice balance between fun and simulation
both games have their good choices of cars to choose, and both have their nice choices of tracks so im gonna call it a draw on this one
sounds, forza wins hands down no question, gt5 cars sound like lawnmowers!
i would say buy both games if you have the choice, but i would take forza everytime cause its far more enjoyable to play for the common gamesplayer as its far easier to play and get into