Funny you should say this, I have been using the Ifrit Codex which has Determination on it and then just yesterday I got the Garuda book that swapped 18ish Det for 22-24ish spell speed (sorry dont have exact stats) and i really did prefer the spell speed, lets just ignore the other stats for now because of course it has more MND etc. There was a noticable difference in my cast time, maybe only half a second but it was still faster which meant i could keep someone up better.

Going back to the other stats, det 18 swapped for +4MND and my heals are stronger by about 50, and i know the strength of the book factors in a little bit, but i don't think det has been that important for me.