I know some of you are getting sick of the plugging, but I'm again going to refer to
my homebrew on this one. We had the same basic issues, because we had three "magic" classes, each of which addressed this differently:
> The "Mutant" class can buy Spell-Like Abilities, useable at will. A lot like the Warlock, but less casting-centric; most Mutants take the class for the inherent boosts (stat boosts, natural armor, damage reduction, etc.) to support weapon fighting, with maybe a single SLA for flexibility.
> The "Channeler" class is a spontaneous caster class that damages the caster depending on the relative levels involved (low-level stuff is free, mid-range stuff is manageable, and your top spells are very draining). The damage is "mental" damage, which can only be healed slowly over a period of a few minutes. (To compensate a bit, the class has a d6 HD and light armor.)
Imagine a Psion who has a small power pool but heals a few power points per round, and you've got something pretty close to the Channeler. In the OP, the #3 option (Corruption) is closest to what we use here.
Given a couple minutes between encounters, a Channeler can always be at full strength for the next one. But, he simply can't unload tons of high-level stuff at once; he has to pace himself. In fact, you really can't use more than one or two high-level spells per encounter, period.
> The "Wizard" class is a slot-based caster with some swapping ability like a Cleric, and is basically a typical D&Dish "per day" class. He CAN unload all of his heavy stuff in a single encounter, but if he's smart he'll pace himself. Wizards really shine in boss fights.
So anyway, we ran into this exact discussion, because we were mixing a "per day" class with a "per encounter" one with an "at will" one. Frankly, most of our players preferred the "per encounter" setup, even though it was weaker during the "boss fight" encounters, probably because they KNEW they had control over their resources, instead of hoping the DM had spaced out the encounters enough to allow 8 hours of rest. But the fact that each side had at least one player who felt it was the "best" was a good sign.
Anyway, if you ARE going to switch to a per-encounter system, I'd really advise against trying to make everything "X uses per encounter", because that still just favors the alphastrike (unloading all your big stuff at the start of the fight) and if you've got a bunch of high-level spells, each with 1 use per day, you STILL come out better than in a normal slot-based system. This is where drain-based systems really shine, IMO.