Choosing between GT and Forza Motorsport 5 would have been a tougher choice for me, but the state Driverclub is in right now, I don't think it's even worth comparing to Forza Motorsport 5. It might be a bit too early to compare these two games and predict which one would be better, but if I have to root for a particular game right now, I would definitely root for Forza Motorsports 5.
Driveclub is still in alpha and nowhere near completion. The devs have just started working on it since last year. So, it would take some time before we can see exactly how well it will play. On the other hand, Forza Mototsports 5 seems to be the next big thing. It is going to set the benchmark for future racing games. I really like their unique take on AI where the game will remember your play-style. It's so subtle for someone as naive as me to comprehend the whole AI mechanism, but I like what I see