Put it this way, the RSW12 should be at least as good and on paper certainly looks so. Given its rrp it should be better, but the BK subs are extraordinarily good for the money so it could be closer than rrp's sugest.
IMO, getting the MA sub for the same money will get you a sub at least as good as the XLS200, and there is a decent chance it is better, so its a safe bet really. Ive not heard the XLS 200 myself, but the RSW12 is by no means a bad sub, and for £300 its a very good one
If you have a bit more to spend though, then the XLS400 would take my preference over the MA sub.