Both ideas will result in greater bass capability, but there are two other factors to consider.  First, a larger sub will typically have better low frequency extension.  There are several very large subs being manufactured these days which have powerful bass response down to 20 Hz and below.  It’s hard not to choose such a sub over several smaller subs when you want high impact and full bass response.  The second issue is response smoothness and seat-to-seat consistency.  The use of two or four subs placed in the proper positions of the room makes the bass better integrated and much more consistent across a wide area of the room.  This is my preference since I believe most theaters have more than one seat and I also enjoy listening to high end music on my system.  The excellent bass integration and smoothness multiple subs creates makes this a very pleasurable experience.