I have compared them. Pianoteq is far superior when it comes to dynamics and responsiveness. Not only does it sound real it feels real. Being a pianist myself, that makes all the difference. If you are a producer and you just wants something that sounds good than, truepiano should suffice.
I have both. As Anthony said, Pianoteq is particularly good if you are a pianist. Truepiano is really good for a solid piano sound with a little less CPU overhead. One of my favorite things about Pianoteq, though, is since it’s a physical modeling instrument, you can do some pretty cool things with it. It will read Scala tuning files and I recently used it to compose a piece for microtonal piano that worked amazingly well. If you try to do that with most instruments, it just detunes the samples with all the artifacts entailed. However Pianoteq retunes the "strings" so you continue to get a realistic piano sound but with different tunings.