I believe the P3's are rated at 500 rms. I'm pretty sure they could take 600 clean watts in a smaller box. I would rather power the P3 adeqately than underpower the 9500. I would get the 9500 if you planned on upgrading power in the future because the 9500 is a much better sub, as mentioned earlier.
my question is, why so big of a box? both won't reallly like that box....would be much better if it was 4-4.5cuft. But off 600w, I'd take the P3...in a smaller box. Now, if your running that 5.8cu.ft box, the the MTX9515 off 600w because it won't even probably take rated power in a box that big.