Deciding which pokemon to power up?

This past weekend I reached level 22 in PoGo and I believe I'm at the point where it's worthwhile to begin powering up my Pokemon (I have ~150k stardust saved up). However I want to make sure that I am using my stardust as efficiently as possible and I've run into a bit of a dilemma:

I have two Flareons, both with the same moveset. Flareon A has 51.1% IV w/ a CP of 1263 and Flareon B has a 95.6% IV w/ a CP of 751. What I am wondering is how much of a difference the 40% IV gap makes and which Flareon I'm better off spending my stardust on. Is it worth powering up Flareon B even though I will have to use a good amount of stardust just to get it to the same CP as Flareon A? Or am I better off powering up Flareon A because of how far ahead it is?

I have many other couples of pokemon with the same problem (ex: 47.8% Clefairy 635 CP vs 66.7% Clefairy 178 CP) but I went with the Flareons for my example because they have the same moveset and cannot be evolved any further.

If this question has already been asked I apologize but I couldn't find an answer when I looked.