I'd say it's a choice for you between Super GT and Cruisn'. If you like the "cartoony" style "driving game", go with the cruisn'. If you want to race, the Super GT is the choice all the way. I personally don't care for it (I like Daytona better), but I gotta say, Sega Super GT is an awesome game. 2-player, linkable, force feedback steering wheel, realistic style racing, and it has a good soundtrack overall. Course are challenging for either single player or dual racing.
Don't get me wrong, this is NOT a "bash the cruisn, hype the GT" post. Cruisin is a very fun game to play as well. I prefer USA over World and Exotica, as those two kinda got way off the wall, IMO. Cruisin has decent graphics, for what they are trying to accomplish, they did a very good job. Repairs are a LOT easier on those cabs than the GT (speaking from experience). Also, Cruisn' is better for all ages to play, as it is a much more "user friendly" game. Kids can pick it up a lot easier, and it's much more forgiving if you hit something, or run off the road.
So, decide on two things. Is this for you, or others? If it's for others, what kind of people are you appealing to? If it's for you, what do you want? It is your game, after all.