The Zoom B2 is a multi-effects unit. The RC2 is a dedicated looper. They are not at all similar products and neither can do what the other one can. If you're looking at loopers, there aren't too many out there. If you ignore the Zvex Lo-Fi looper which is a bit different, then the Boss is the smallest dedicated looper you'll find. Digitech are launching a similar single stompbox unit but we have no date yet for when it will get to market.
You could also look at delay/loopers, of which the Akai Headrush it one. Although the Headrush is a nice, simple unit, none of the delay/loopers will allow you to store multiple loops like you can with the dedicated loopers and their recording memory will be limited, normally to 30 seconds or less. Other delay/loopers are the Boss DD-7 and DD-20, Hardwire Delay/Looper and the Electro Harmonix Stereo Memory Man with Hazarai.
For dedicated looping also look at the Digitech Jamman and Line 6 JM4. But these are bigger and more expensive units than the RC-2.
If you're close to us (Northwood, North West London) then pop in for a demo.
Richard