i think each city has it's own club, but they're linked together. i know we've had different rates in the different areas we've lived in. is it possible to find the least expensive club and join that one? (doesn't sound likely....)
also, if you're just going to join solely for the room discount, just get a single membership--no need pay to have all drivers covered.
if you do join, get them to mail you the local maps. there's a GREAT inset in one of them (i think it's orlando vicinity south) that shows all the roads within WDW property. very helpful!