Hi, I have a lot of BK P-90s (including a Pig-90 and 2 MQ's, plus 2 regular P-91's and a P-90 rewind!). THe Pig-90 has more bottom end than a regular P-90, it is not quite as raspy as a driven P-90. The MQ's are P-90's but in humbucker size covers and so won't work for you. THe only problem I had with the Pig-90 dogear, is that the metal cover is for an archtop not a junior, so I had to do a little routing to fit it (I put it is a DC Junior copy so I didn't mind the work). You can get it in a regular cover though.
If you like early Black Sabbath sounds (from the 1st album), then the Pig-90 is great. If you are looking for more of the Mountain or Neil Young sounds, then go with the P-91 or 92.