I briefly tested a PT-7P at ITEC this past May. Too me the PT-7P felt a lot like my PT-6P. One difference was that it was definitely physically heavier and the sound I got out of it kind of reflected that. It wasn't that drastic of a change though from my 6's sound. Beside that though it wasn't too terribly different. I couldn't really tell you about other 6/4 tubas in comparison.
I would definitely recommend the PT-6P though if you were thinking of buying a big CC tuba. I love mine so much I'll probably never give it up.
I have tried the PT-7P in comparison with piston Neptune and Baer. Compared to the Culbertson Neptune the PT-7P has slightly smaller diameter bell (500v520mm) and some of the valve tube wraps are slightly different. Playing wise they are very similar with the Neptune being a fraction broader. Both are bigger than the Baer which felt half way between them and the PT-6P.
Personally I would go for the Neptune over the PT-7P because it looks better (to my eyes) in the gold lacquer and I believe costs slightly less. However, the PT-7P is a nice playing tuba with gorgeous tone and would be high on my list to consider if I was looking for piston 6/4.