It is hard to make a choice, but I totally forgot to point out what Matt did with the fact that Epik has been backlogged with their orders so there's no telling how long you would have to wait...
Now as for the dual MFW-15's, that is a great and more cost effective option for duals subs if that is a route you really want to go....I know when I was researching my sub purchase that I read that the MFW's (dual MFW's that is) will have about the same or the slightest edge in terms of output on the Ultra. Of course dual Ultra's will be over the top....
Really comes down to the amount of space you have and what you are looking for. The box for the MW's is even bigger than the Ultra if I remember correctly, and that just was not an option for me--not to mention that I get the same out of one Ultra. As for the driver size, it holds up as I went from a 15' to this and it walks all over my Velo 15" that I had previously...
Despite the fact that I do own the Ultra, I have gone through a decent share of subs and can tell you in terms of output, response, sound quality, build quality, and customer service the PB13 is bar none the best that I have had the pleasure of owning...