Either one would be fine by me, as long as GT had one I would be happy.
*Gratuitous Forza even had it picture*
Why? Mainly because I drive a G60 everyday in real life and i think it would be awesome to drive one around in the game. Also they are great handling cars so driving them around the 'Ring would be awesome, the sound track that goes with it would be great as well.
On a side note, that isn't me in the video... i would never thrash my car like that.
That's one of the better choices as it is a car manufacturer no one really knows of, and make cars that are really decisive between good and ugly in design.