If you're looking as a developer you're best to get a phone that is not application locked or SIM locked by the carrier. That would mean something like i-mate or Qtek.
The next thing that matters is what you're planning to develop in and who you're looking to target. Are you using .Net C# or VB and .NET CF2? Then you're going to need a WinMo5 device. If you're targeting older devices, then you need to consider the trade offs with getting an older device and not being able to test against the newer devices.
Lots of things to consider.
