My first Chris Christensen was an oblong 20mm pin brush and I used it for a couple years, then the pins started pushing into the pad. When I replaced it I bought a 27mm round fusion brush. Well, the head seemed too big for my Yorkies and hated for all that money to go to waste, so I tried it on my own long hair. I love it! It works great and feels good on my scalp.
I bought a new 27mm oblong brush for my Yorkies. I find that with a long coat, the 27mm works better and the oblong size seems to fit better. These are great brushes and are worth the extra cost. I don't see any benefit for the fusion though.