At range <=8, Ph-1 are more energy efficient than dizzies. You'll get more damage per point of power.
At range >8, dizzies become more energy efficient.
However, if you are looking at a down shield, IMHO you need to hit him with everything. At range 15 6 Ph-1 will do 6 more internals.
If he has a shift on you at that range, or if you need them for PD, you can hold the phasers. Otherwise take the extra internals.
Keep in mind also, dizzies cost the same to arm and hold, so you're not draining any additional power by firing them. You pretty much want to fire them every chance you get.