I played an undead arcane mage. So technically that puts me on the Horde side. But the only Horde races I actually liked were the Forsaken and the Blood Elves. The others seemed kind of uncivilized. I was kind of amused to note that the reputation I started out with towards the various Horde factions mirrored my own prejudices rather closely.
The way I ended up choosing my character was kind of funny, though. I was curious and asked myself, "how could they make the game make sense if you're just a marauding, mindless undead creature that shouldn't have any will of their own?" Then I happened to glance over at the Forsaken on the character selection screen... and I noticed that they weren't moving quite like typical undead, and showed signs of being deliberate and conscious in their movements. One particular idle animation of the female Forsaken model, where she glanced around nervously and seemed wary of the environment, reminded me of myself.
The more I played them, the more I related to them. Their main enemies (the Scarlet Crusaders), are shockingly similar to the people I perceive as my enemies (certain right-wing cultural elements). The idea of building a city in tunnels underneath the ruins of a walled city makes far more sense to me than the locations of the other cities.
Anyway, sorry for that tangent. My point was, I think the Forsaken and Blood Elves could reasonably be Js, even if the rest of the Horde is P. So I don't think you can determine things solely by faction. I know I wasn't even paying attention to Alliance/Horde when I created my character.