General Elessar, since this is for good lists, I'd suggest you think of it this way: Gandalf is better for heroes combos (especcially Elven ones) where he can keep key abilities charged. Otherwise, Saruman adds something good lists seriously lack, mobile firepower and something to buff them in combat. 'Shatter Shields' can be highly consequential just on its own, and Saruman can also call Epic Ruination, which means that he actually dishes out quite a lot of damage.

Good Saruman is 200pts, with courage 7, 3 might, Epic Ruination and channeling, and 3 spell lists. What he lacks is overlord, inspiring leader and touched by destiny.

I think you need to have a very specific plan for Radagast and he fits some lists better than others. For a long time, he was the only Istari I owned, but I have aquired a Saruman and liked him. I gave Gandalf a try and was unimpressed. I was unimpressed because I simply found him a bit redundant for Gondor, but would change my tune if we were going up to 2000 pts (1500?) and Gimli joined him in the allied choices Saruman, by contrast, is nice and simple and fits Gondor well - whereever he goes he can significantly boost kill output on your core infantry, while also have extra tricks up his sleeve.

Radagast seems best for Rohan and Elves. A slight disadvantage he faces is that 'Epic Tranquility' seems to get some people inordinately riled up. I don't see why, since there are a number of non-melee ways to get at him (and there is always heroic charge), but there you are.