Generally it isn't a good idea to use something important like your primary phone as a test kitty.
That being said I'm planning on doing the same stupid thing, so +1. :-)
My feeling at the moment (no pun intended) is to go with the Samsung Moment or Motorolla Droid phone, as they have a hardware keyboard. That may come in handy in a pinch if something goes wrong. Flurin in the comments mentioned that Motorolla has a better track record with providing OS updates, so you may want to keep that in mind if you have a choice of carriers.
Idea 2 would be to go with the Google phone. It has the most up to date version of the OS. The theory here is that if you are shooting at a moving target, it is best to lead the target a bit. However, I'm on the Sprint network, so that isn't an option for me.