The FN holds 2 more rounds per mag, gives you more choices in how the pistol is kept loaded (condition 1 or 2. any XD only gives you the option of condition 1). But I haven't fired a FNP-45. I do own a FNP-40 and a FNX-9 that I like... however, my XD45 sits on my nightstand.
My take: get whichever one you damn well like better. They're both excellent guns. If you like the way one looks better, or can get a better deal on one, or one feels better in your hand, or you feel like flipping a coin, whatever. You can't make a wrong choice here. About the only objective differences are that you can decock a FNP a not a XD, that the FNP does not have a grip safety, that the FNP comes with more mags, that the FNP holds more rounds per mag, and that the XD has a better warranty (lifetime vs. 3 year for the FN's). Oh and the XD comes with a free mag loader.