In reply to Papasean:
I was in the same position a few years ago when I moved from England to Scotland.
Having been members of both, I'd say I get the most value from being a MCofS than BMC.
Apart from the fact I paid £17.50 for 1 years membership this year and got a £30 silva exped 4 compass as a free gift; I'd say that MCofS does a better job of representing the rights of Scottish hill goers and provides better information throughout the year.
The BMC is still very much a 'climbers' orientated organisation which is fine if that is your main activity, whereas MCofS is more walking/mountaineering orientated.
As for discounts, most shops give discount to both MCofS and BMC members, the only ones that don't are snow and rock, they don't recognise MCofS for whatever reason, possibly as they have no stores in Scotland. Hopefully now that Cotswolds have taken them over they will recognise that there are one or two mountain users over the border that may be interested in shopping with them. Also tiso's don't discount anyone because of their daft discount card you have to pay for.
Anyhoo, I'm getting sidetracked! Now you are in bonnie Scotland join the MCofS
that's just my 2 cents