Personally I'd prefer something in between. Let the player have a limited set of selected spells that they can cast per encounter, then add in a magical focus that provides additional spell themes -- say up to 1 additional spell/level, depending on the power level. (The focus would be an equip-able item like an amulet, diadem, tome, or ring.) If the player doesn't like a focus, they can find a new one and become attuned to that. At least then you'd have some additional tactical variety and advanced planning.