The Monolith is a very different sub to the B&W.
True sub enthusiasts would say the B&W doesnt even come close to the performance of the monolith which it wont - the monolith has a flat freq response down to 20hz with no boosting or EQ to do it - its a proper sub based purely on design and cabinet volume, that will mean clean bass right the way down in the right environment (That wont be your listening room unless you are 5 x rollover lottery winning lucky). It wont matter whether you buy the FF or DF it will perform the same.
However it depends on what you want the sub for - obviously bass but...
The B&W is a sealed box sub, so easier to place and probably faster and better for music - emphasis on probably - however It wont go as deep as the Monoilth. A better and cheaper option is the BK XXLS400 - very, very. very good sub for the money, is better than the Monolith for music but again it doesnt go super low but is a good alrounder. There are reviews online about the XXLS400
The Monolith is more aimed at films / movie sound tracks where its really going to impress and will hold its own with subs costing 3x as much, actually more probably than that. It is also a very popular and a very good sub with lots of online reviews and opinions - and obviously its BIG!!
But its the same with all subs - unless you can get the placement and performance keyed in then it doesnt matter what sub you buy the bass will sound the same - bad.
You wil need some bass management (or room treatment) within your system - if your receiver doesnt have it built in - Audessey / ARC - I would check it I were you - Then budget in for an Antimode as they really do a very good job, they are the next best thing