Not 100% sure, you haven't had any replies though, so I thought I would give it a bash. I am part of MCofS, you get a MCofS magazine every quarter, instead of the general BMC one, so if all your climbing is in Scotland, it might be more relevant for you?
Other than that im not aware of any major differences. You get liability insurance with MCofS, but I think you get that with BMC too. Hopefully someone in the know will chime in.