For the life of me, I can't understand why you would want a bigger pad than 6".
I've used 6,7,9, and 11" buffers in my fairly short detailing career (15.5 years), and the PC is the perfect size for me. I like the fact that the 6" pad is very manueverable, extremely controllable and that it allows you to see exactly what's going on at all times.
When I had bigger buffers (still have most of them), I felt like I was driving the Titanic in a kiddie pool. I was always slopping product over trim pieces, etc. Plus, I had to really push on the d@mn thing to get any decent results (if working with a neglected finish).
The 6" pad allows me to sneak under mirrors (on door panels), get close enough to the trim pieces with out getting sloppy, and I can cover large surfaces with great control and precision. I have 3 PC's, and one is almost 6 years old & still going strong!
For my money, nothing beats the PC. Sure, you can't cover the car in one pass, but I'll take finesse and control over brawn and mass any day!