I've heard good things about the Proxes. The proxes are an all season performance tires whereas the nittos are summer performance ones. I live in Mass so I don't drive the mustang during the winter months. I found the nittos to be more suitable for my application. I think the only drawback is that the nittos will wear faster.
I had 245 Proxes4s for a long time, and liked them so much than now I've got them in 275s on my new rims. You will not hydroplane with them, it's kind of crazy. Only problem with the 275s is that the treads are so huge that I kick up HUGE rocks on the road.