Hi Mel,
There are a lot of factors to consider, like Darren mentions. Accuracy, price, ruggedness, usability, applications, and the list goes on.
What sort of accuracy requirements do you have?
What is your budget?
What applications are you hoping to use (on the device and/or on the desktop side)? Do you have access to a server of sorts (like ArcGIS for Server)?
Who will be using this (experienced GIS people, regular field crews, or anybody and everybody)?
What kind of ruggedness do you need (will an iPad do or do you need one that is water-proof, heat-proof, dust-proof, fall-proof, Class I Div II, etc)?
How many units will you need?
Those are some questions to consider.