Side by side, what's on the disc and downloadable, GT6 is better. Better track selection and obviously some newer cars added. The C7 Vette is tunable and I have 40 in my garage, and all different. Some features in GT5 were better, such as the ability to delete paint chips that you don't want, but in GT6 you don't have to buy another car to gain. 5 is good, 6 is better.
Custom tracks were better in 6 until PD totally dropped the ball and did not figure a way for us to keep when servers were shut down. Can still build in 5 but no longer share. To me, of all the GT games, this was the biggest fail.
But just wait until Sport has it's servers plug pulled.
Comes down to opinion, I like 6 and continue to play even now. And I don't HAVE to do B-Spec.