We develop our own survey data logger, PocketDTM, see
This runs on most CE and Windows mobile devices, and we have recently host it onboard on the Leica RX1250.
In terms of hardware, the TDS recon is a pretty good price performance combination if you don't need a keyboard. They also have a numeric keyboard device, the Nomad, and full alpha-numeric device, the Ranger.
As the previous post suggests, you should try a few different devices and software solutions to see which suits best. Most software vendors will provide a time limited trial version for free, and in the current economic climate, many suppliers will also provide loan hardware.
Things on your checklist outside of software functionality should include screen readability, IP ratings, battery life, access to removable storage, and bluetooth.
Atlas Computers Ltd
Survey Control Center & PocketDTM