Both have great features. I have both.
I think if I were to get another P or 15... I would get another 15. They have games that I like better, characters and its limited ed. The cost will only go up on them in time. Toss one in there that its the 3rd IDL, making it the "rare" edition.
P's are still being made and the price seems to becomming steady. With 3 different sets of P's being out already, they might be easier to get later. Original, Melody and the people one... Akisu or something. The last likely being the limited version.