Yeah I agree that both are likely improvements on the B&W sub, just saying also there's more than those two used subs out there. Take your time, look around for a while. Listening/audition sessions for subs are limited, too, as much depends on the setup of the sub in that specific system and room (the room being a huge influence, thus why many use particular eq applications to set up well). Dual subs make it easier to tame a room so to speak, provide more even response thru the room, but a good single capable sub, particularly if only needing to worry about your own seat, can do well, too. Can't make the decision for you, tho, just provide some information and things to think about....
As to the room's influence you might try this article
https://www.soundandvision.com/content/schroeder-frequency-show-and-tell-part-1