The Atrix is a very capable and fast phone. I love mine. The Atrix is a dual core phone. The Xperia Play is a siglne core device. However the Xperia Play has a game pad and the Atrix doesn't. So it basically comes down to if you are going to play games on your phone that require a game pad or not.
It depends on your criteria of a better phone. I suggest you take a look in this comparison of Motorola Atrix 4G and iPhone 4G found here in Cell Phones etc.