I prefer belt primary and final drive for most of my bikes because of low noise and maintenance, no oil slinging and alignment isn't as critical.
Chains offer more efficient power transfer and don't stress your bearings as much as a tight belt, so I may use chains on my next race bike. I have one bike with belt primary and chain final.
I also have one with chain primary and final.
After the last race at Grange, I'm seriously considering a friction drive racer.