If you can afford it, get the RB. It's top quality GK pro gear. The 700RB210 sounds fantastic. You can expand it later if you want by adding another speaker cabinet for even more output and coverage.
The Backline series is low priced beginner stuff made to compete with the other beginner stuff. It's not bad for the money but I wouldn't use it for playing live, with the exception of the Backline 600 head if used with better cabinets than the Backline cabs.
I'm a big GK fan and have used their amps for many years and never been let down.