how can you honestly answer that question, the yzf isn't even out yet. i personally would go with the yamaha because the motor has been proven to be reliable.
I agree with JustinC, Yamaha has been reliable. The KTM is probably going to have more power. The KTM won't have the aluminum frame like the YZ-F. I say go for the Yamaha for over all better performance, and probably reliablility. This will be the first year for the KTM, so I would wait on that one and just go with Yamaha.