There will be very little difference in brightness, especially if you get the Q5 special P1CE. I get the impression that the P1CE has some build-quality improvements over the P1 as well, so I'd definitely go for it if I had a choice. I'd consider an SSC mod if I already had a P1 so it was just a matter of swapping the leds.
The P1CE is certainly the best-built Fenix that I've owned so far (of L1P, L0DCE, L1DCE, and P1CE). But I've never had a P1 to compare it to.