I had a writeup about the two some where earlier, but it kind of matters what you'll use it for. I know alot don't like the Fender stack in the SS and SSDI, but any pedal I put in front of it sounds amazing, clear, punchy, still tube but not lazy, and powerful, that's probably the best work to describe it, powerful. I find the SSDI works best for me in recording and practice setups.
The P15 is a whole other animal, vintage, warm, you can get considerable grit/breakup (if you want), extremely interactive EQ, especially cool with limited controls. P15 help Broughton onto the map, and it's amazingly fun to play, but with the 2 I do reach for the SSDI for the flexibility of what it can play most of the time. You can't go wrong with either, the components inside make it so big power is always available, a lot of little subtle differences that equal huge sound with excellent dynamic response. I would suggest getting both, but if I had a choice I'd get a SSDI and Fliptop if the $$ wasn't there, that combo sounds great as well for a vintage model. Theres clips around here, one I could remember is on page 2112