First, check out hte AM Acoustics room mode simulator. You can use it to pick ideal locations for your listening location, speakers and subs as well as evaluate where your lowest room modes will be and how to fix them.
Personally, I'd use one, on the basis of cost, foot print and performance. I've had nearly perfect results in some situations with a single subwoofer. Having put that sub in, dealt with room modes, added appropriate EQ and bass traps I would then ask if I needed a second.