The whole crit/rng argument gets pretty much destroyed in long fights, which is where spellspeed is good anyway. Over a long fight the amount of crits you get will tend to your critical hit rate, especially if you have a large amount of spellspeed.
Determination is the lowest weighted stat, and gives you the lowest amount point for point. At i100:
Chest/Legs:
Main:27 det/38 crit/38 spellspeed, Secondary: 19 det/26 crit/26 spellspeed
Head/Gloves/Feet:
Main: 16 Det/23 Crit/23 Spellspeed, Secondary: 11 det/16 crit/16 spellspeed
Waist/Jewelry:
Main: 12 Det/17 Crit/17 Spellspeed, Secondary: 9 Det/12 Crit/12 Spellspeed
As you can see, you get far more stat out of crit than det. Its fair enough if you choose not to run it and run determination instead, as you can run with whatever you want, but it gives far more stats than it does determination, and is weighted higher.