i own the JamMan and have used a RC-20XL and i can say i'm glad i went w/ the Jammer...the USB is nice and the compact flash gives you more options for storing loops which was the big issue and ultimately led me to it...the RC holds what, 10?...no thanks...
the one thing i can say the RC has going for it is the reverse feature which the JamMan lacks but, that's kinda gimmicky anyway...and for sure being Boss helps it's cause as well...prob. why more folks don't have JamMans...