That's a tough one. I'm going to get both along with a few others. That said their still seems to be some confusion as to whether Gravity Rush is going to be a launch game. Assuming they are both launch games it all depends. Uncharted is a proven franchise although this portable installment is being developed by Sony Bend and not Naughty Dog. That said they have a good reputation and the game itself has got good feedback so far. Gravity Rush is brand new and unproven but the preview coverage so far has been very positive. It looks really interesting and fun to play as well as being a fairly original concept and definitely worth taking a risk over. Personally if I had to choose I would go with Gravity Rush. Then again I've played all three Uncharted games on the PS3.
Well we pretty much know what we're going to get with Uncharted already. A great story and probably the same type of gameplay as the other 3 games in the series, so expect a solid game. Gravity Rush is a complete unknown as of right now. There's a really short demo on the Japan PSN store, but there really is no telling if the game will live up to the hype with the story or gameplay. I really want Gravity Rush, but I'm worried that the game won't be as good as its made out to be. I am getting both as soon as they both launch, but since you can only pick one I would say go with Uncharted since that is confirmed for a day 1 release. Gravity Rush has a launch window release which could mean a week after launch or a month later.