My vote goes for 64 Studio which gave me the best realtime performance / low latency out of the box without tinkering. It comes in 64bit and 32bit versions and is based on Debian too.
There is an forthcomming Ubuntustudio project which should give better performance as stock Ubuntu but they are not finished or?
Cheers,
Malte Steiner media art + development -www.block4.com-