I have GTP3 - but not Sonar. I upgraded from Music Creator, and was a little dissappointed that GTP3 didn't support midi - which what I used to create my drum tracks. But now I use drum loops and I am very happy with the sound loops provide.
I can't say for sure, but I think GTP3 is just a watered down version of Sonar. It does everything I want - so I'm happy.