Asphalt GT is definitely worth 5 or 10 bux. Just a standard arcade racer that doesn't do anything too daring, but doesn't screw anything up either (i.e Ridge Racer). At the end of the day, its a nice rush of fun for a portable arcade racer.
As for Mx vs ATV, haven't had much time to crack into yet, but I'll keep you posted.
Never played Chou Soujuu, but KDR should be able to fill you in.