I like magic to be unpredictable, since I feel that best reflects how magic is often portrayed in fantasy stories. It’s not as if you know the powers of the enemy magicians beforehand. Even his own side should and could be surprised 😉
In many games we therefore adopt a ‘random spell draw’. Spells are drawn randomly before the start of the battle – you keep them secret – and it’s up to the player to make best use of them. This also means magic effects are somewhat limited – not huge poisonous clouds that forces all living creatures to make a save vs death or something similar. Such an approach works of course better for game setups that are more on the narrative side rather than the competitive side. YMMV.
If magic is a very predictable game effect and/or simply becomes part of the force selection pre-game mechanics, it loses much of its flavour.
Tiny Tin Men Blog: http://snv-ttm.blogspot.com/
Wargaming Mechanics Blog: http://wargaming-mechanics.blogspot.com/