Maybe you could do both? A lot of the really expensive controllers use both to control when it is injected. Sometimes it will have to be above 950F and 5psi to come on or whatever you want. The reason that some of them choose to use EGT's as well as boost or instead of boost is so that you don't go through so much water. At least on my truck if I had it set to inject water/**** by boost alone at lets say 5 psi, it would be injecting it almost the entire time that I was going 70-75 down the interstate, I would have to have a 30 gallon tank for water/****.
I have been working on my setup too, but I don't have a gauge that will allow me to control by EGTs so I am trying to find another way to do it cheaply.