This is a great question, and I think they are about as even as two up and coming young players can be.
Before the playoffs I would have said George in a heartbeat, but to me the story of the playoffs was the very high level of basketball Leonard played in the finals.
There was a lot of talk about the seven hall of famers going head to head, but Leonard was better than six of them, and by a pretty wide margin I'd say.
Both George and Leonard are young and have a lot of room to improve. I think they will both be perennial all stars. Gun to my head I will choose Leonard. I think he is just hitting his stride and has just a little more upside, with George being better right now. Right now Leonard is paying his dues, but as Duncan and Parker age, he will be the featured player in the Spurs offense as George is for the Pacers now.