Sonnie has a good point here: if you're going for more than one sub then, in order to get the best frequency response, you may have to place one of them in a location which may seem odd at first.
It's very difficult to predict where you will have to place your subs. I have heard calibrated setups with several subs (SVS Plus/2 for instance) in large rooms that sounded incredibly good, impossible to locate the subs and the sound was terrific. When we tried to calibrate the same system (in the same room) with only one of them then we could hear that we were missing something and locating the sub was easy.
So if you have the money then go for two
In my opinion, its better to have two relatively smaller subs than a huge one.