To help you make your decision easier, which type of person are you.
1. Hook up the subwoofer, fire up the equipment, play some demo clips and trust your ears. Wow, that sounds amazing and blows away my old subwoofer. If this sounds like you, buy the largest single subwoofer that fits your budget and then STAY OFF OF THE FORUMS!!!!!
2. Hook up the subwoofer, fire up the equipment, play some demo clips. Wow, that sounds amazing and blows away my old subwoofer. I wonder if I can tweak it to get even more out of my system. Head to the forums, read for hours, order MiniDSP microphone, install REW, measure, tweak, measure, tweak, obsess over a slight null at 40 hz, move sub, measure, tweak, measure, tweak, obsess over a new slight null at 55 hz, repeat, repeat, go back to forums. If this sounds like you, order 2 smaller subs which fits your budget so that you can make the graph look prettier.
Of course, if this sounds like you, the rabbit hole gets deep quick and you will be looking to upgrade the 2 smaller subs in the future.
Once you get past your current question, you will have to answer the next question:
Sealed, Ported, or Low Tuned Ported