Are you going to have access to a second Digimon for battles (either from a friend or a second for yourself)? I have both devices but I haven't had time to play my Digimon X a whole lot yet. I do like that you can freeze it (pause) easily when you don't have time, and overall I found it fun to play. The Pix is a lot of fun. I read a lot of bad reviews about the Pix before getting one for myself, but I have not had any bad experience and like it a lot.
Have you ever had a Digimon or Tamagotchi before? If you have had one maybe choose the other? I think both are fun and it is difficult to recommend one over the other. Really though I would recommend both! Get the Pix now, and the Digimon later (or vice versa)!