radagast, as cheapest(point wise) of the three is very handy. wat he has that the others don't have: Line of sight to the whole table. if he's in range, you can get him. heal: if you've got a expensive hero that you don't want dead he's your man, and panic steed, quite usefull to unmount the leader of a bunch of cavelry, and so take the sting out of their charge. quite usefull against mumakil too
difference between grey and white is spell power. white casts most spells on 1 less (ie 3+ instead of 4+). this does make him a lot more expensive though. i think he has a superior stand fast too
maybe consider saruman(the good one) too. can cast sourcerous blast on 4+ and has a pretty nifty stand fast. no-one in range is going too run for it. he has less spells then gandalf but can cast them on better values for less points. he's my personal favorite.