The 7 is a upgrade over the 5, and in my opinion a big one. I can not compare the delrin platter to the ceramic, as I have not heard the delrin.
The ceramic is certainly part of the sound of the 7, and I think more so on the 9.The 7's platter was after all a way to offer some of the 9's sound at a lower entry point. I have had my 9 for some 2 years now, and have no interest in changing decks. The one thing that is obvious is that it lets whatever cartridge you mount on it sing as it should. That has applied from the dl110, through Dynavector (20x and 17D3, Benz Glider,and the apheta.
At one time I had a P25, with offboard PSU, groove tracer sub platter, acrylic platter, and a rewired RB600, not a cheap route to a good deck, the P7 is just so much better its hard to describe, and the 9 is better then the 7, by how much, I dont know, I would say the 7 gives 85 to 90% of the 9.
I think the reason you see more 9's for sale is the simple reason that they sold more in the first place.