As a person who have completed both games, I'll say go for Fire Emblem.

Not to say PxZ isn't good or anything, but the gameplay can get extremely repetitive and the later levels taking 1-2 hours or more to complete. But If you're a fan of many of the characters that appear in PxZ, you may wanna try it out, but I recommend checking the demo out (there's two PxZ demos available).

Fire Emblem has a better appeal (with this game having a casual mode to cater to newcomers of the game), and tons of replay value, as well as some optional paid DLC as well as free DLC (Spotpass content).