Which is better?
Looking for max spl level ratings at 15, 20, 30hz.

"Better" is a tough question. I've only actually heard the PB13 myself, so I can't say.
As far as Spl goes:

Spl at 15hz: 1 Ultra 13
Spl at 20hz: Pair MFW-15
Spl at 30hz: Pair MFW-15

Dual MFW-15s will give a little more output above 20Hz compared to the Ultra, but the Ultra can dig deeper. Better is going to be purely subjective. I can tell you the Ultra makes me smile. ;)

If a single PB13-Ultra can deliver more output than dual MFW-15's down to 15 hz then I think it will do so at progessively higher distortion levels.

--Regards,

I'll get the ultra.