I've never owned a Phat Cat but I've heard lots of clips that sound very good. Because of its A2 magnets I always thought it would sound too "polite" and I never got excited enough about that to lay down the $$ for one. But the clips I've heard actually sound very alive, bright, and punchy like you'd expect a good P-90 to sound like.
I've got lots of P-Rails, lots of P-90's from many different brands, and I've got lots of P-90's in humbucker size. There are actually a lot of differences in their sound and performance, even comparing them with the same magnets. They all sound good (I really love P-90's) but the P-90 coil in the P-Rail is one of my favorites, no matter what magnet I put in it (but my preference is A5).
The reason I've never owned a Phat Cat, and thus never made a direct comparison to the P-Rails, is because the humbucker-sized P-90 pups from GFS are so great and only about $30. Their Mean 90 used to be my favorite due to its full-bodied flavor with big mids, but I've grown more fond of their Dream 90 lately because it sounds brighter and more articulate...more like the P-Rail's P-90 coil but with less low end. I think I still like the P-Rail slightly more, but they all sound so great how can you choose?!
Oddly enough, my least favorite P-90 was from a Gibson LP Goldtop. It just sounded bland and blah no matter which magnets I put into it.
Anyway, let us know what you think of the P-Rail compared to your Phat Cat. The REALLY great thing about the P-Rail (in addition to its super P-90 tone) is the tonal options available because of the additional Rail coil.