I rather like the idea of at-will spells for things like cantrips and lvl 1 spells, then semi-Vancian spells for higher levels.
By semi-Vancian, instead of "memorization" just say that it requires advance preparation. Don't have spells be instantly "forgotten" but rather that they need a framework, laying down of energies, some kind of fluff to make the wizard actually feel like he's not just choosing specific bullets for his gun each day. The vast training required for magic use requires something like knowing how to handle the power flow of magic, understanding how the mind interacts with magic, that kind of thing.
But actual "forgetting"? Meh, forget that.
And perhaps as wizards, clerics, or whatever spellcasting class gains in their levels, open up spells to becoming "at-wills".
When you're level 1-4, only cantrips are at-will. At lvl 5-9, your level 1 spells become at-will. At level 10-14, your level 2 spells become at-will. That would also speak to the advancing power of the character and show that "simple stuff" becomes routine.