Well using Blizz III gives you an instant 3 stack of Umbral Ice, as opposed to Transpose which is only 1 stack. The more stacks you have then the more MP you recover (in Umbral) or the more Fire damage you deal (in Astral). Generally it's recommended to avoid Transpose unless you can't avoid it i.e. you need to dodge an aoe and casting isn't an option etc, so most rotations out there will have you maximising Blizz III and Fire III to keep up those stacks in between switches.