True, Im a long time advocate of the Atronach sign, but I wander how fast 100willpower is.
http://www.uesp.net/wiki/Oblivion:Willpower
it depend on you magic pool, as I understand it uses 100/2.75= 36 seconds to refill you magic at 100 in willpower, so it's fast enough outside of intensive combat.
Personally I would say the apprentice is better for two reasons, magic is not the big killer in the game, plays a pure mage apprentice now with 40 in endurance so she has low health but after level 10 enemy spell casters are pretty easy to handle, main exception is lich with reflection.
But the main difference is that if I played a Atronach I know I would use far less magic as I had to work or use up resources to restore it, kind of take away the purpose with a pure mage, the 1 sigil stone enchantment you have to add does not make up for it.