In your case, if you can place each in an ideal location then do two.
I don’t believe that smaller = faster, at all. That’s a misunderstanding of what really happens, however if you are flexible in the sub placement so you can smooth out the room response, two will give you the better option.
Also, consider bass traps in the corners and a miniDSP to equalize each sub separately, and plug the ports. Set the subwoofer as high as your measurements say you can.
The miniDSP site had application notes for EQ'ing multiple subs in a room.
Check out the AM Acoustics Room Mode Simulator, it will help you place your speakers and subs outside of the worst and lowest room modes.
Of course, another problem often faces is that a reflective room with all parallel surfaces will be bright. Treat it as needed first, that will help the bass bloom. Then you can set your subwoofer level correctly. Don’t forget the ceiling between listener and speaker.