Unnecessary choice. Buy both.
A truly mobile gamer needs both, a cellphone and a handheld console. If your phone happens to have a gamepad and good hardware that allows you to play even more games, it's all for the better.
Not only that, but a duo like this will allow you to tether the s*it out of your Xperia Play as you're playing on your NGP/3DS online. No longer will you be confined to your gaming groto with your handheld - become a walking hotspot.
Isn't THAT a good deal?