I would definitely go with the largest ported enclosure your room would allow, though much over 5 ft^3 is starting to cross over into a region of diminishing returns.
Here is a quick comparison of ported vs. sealed for the Dayton RSS460HO 18" driver. The ported configuration really gives an extended bottom end compared to the sealed. These were both modeled in 6 ft^3 boxes with 600W power.
You also will get improved bass response in your room with multiple subs. 2 smaller subs are usually better than just one large sub. 3 or 4 is usually sufficient to achieve smooth bass response throughout the room, IF, they are placed in appropriate positions.
Last edited by mhutchins; 08-03-2014 at 12:22 PM.