In reply to matta:
Personally ou of the 4 i would spend slightly more and avoid the vango ones, might just be me being snobbish though. Plenty of the vango stuff we use at work doesnt ether last that long or offer what it promises.
I have a rab ascent 500 and its an amazing bag, kept me warm in freezing conditions in africa, it packs really small and is pretty light. A perfect 3 season bag really.
The alpkit ones are meant to be really good too though i dont own one.