If you can afford it, go ahead and make it a double --- Atrix and Play.
If you're into games, there's going to be some games specific to the Play that will make use of the keypad, etc,. and its all Sony's. On the other hand, there's going to be some games that is going to be specific for the nVidia Tegra2, meant to exploit its sheer power. That's for the Atrix.
I enjoy keeping multiple Android phones because they constantly sync with each other to maintain the same contacts, email and calendar base.