In reply to BnB:
They're all pretty much the same in my opinion (apart from teh Terra Nova Superlight Bothy, which is, well, light!).
I have a trekmates one, the MR team uses Aiguille, and if I wanted another I'd buy one of these:
http://jdscomponents.co.uk/bothy%20offer.asp
A four person may be better for you as it would get more use, but obviously weigh more. If there's two of you in you can just pull it in closer/sit on the excess a bit more.
The other option, if you are bothered about weight, is two 2 person bags.
Far more useful than a foil blanket (which I don't carry) and don't think about them for emergencies only. They're good for lunch stops in poor weather if you can't find any shelter, and a good moral booster.