I really like this game, but having Persona 4, a brand new PS2 controller, and fancy component cables for my television readily available for use is starting to bother me. I picked up P3P on a PSN sale, and have really enjoyed what the game has presented me with so far, which is a game I'm sure to finish, but would starting Persona 4 be a poor idea? Fifteen hours is a decent amount of invested time as far as JRPG openings go, but I don't think it's enough to feel disappointed if I were to put it on hold for another game. Persona 4 accompanied with new PS2 equipment is really begging to be played.

Any thoughts?
 
(no spoilers, obviously.)

I say stick with Persona 3 for now.
 
It's worth finishing.

I think both games deserve to be played separately. Each game, despite similar gameplay, have a very unique tone that you should experiment fully, without mixing. 
 
Still if you feel tempted go for it! 

P3P has cameos made by P4 characters.  Though then again, P4 has P3 cameo events as well.  Its all up to you, though I suggest sticking with the game you already started.