Theoretical Scenario: If you are fighting Twintania and for the whole fight you had to cast Cure 1 without ceasing and not have to worry about serious overhealing or movement you would cast Cure 1 about 308 times, assuming you complete the fight as fast as 12 minutes.
With lesser spell speed, the number would be closer to 294 (let's use this for det and crit build)
In this theoretical situation, Spell Speed would WIN by about a single percentage, increasing more the longer the fight is. However, there is not a situation in the game where this has come into play, as you need to move around frequently in these fights, making constant, uninterrupted healing impossible, and thus Spell Speed, pointless.
Consensus: Gear however the hell you want between Spell Crit and Determination. The difference between gearing for spell crit or determination for a white mage is small enough to be completely negligible.