In my mind, it really depends on 'how' you will be using the scope. If you will be using your reticle for elevation, I'd select the 3.5x15, as it is only corerect on the highest power (either 15 or 22). When you are on the highest power you will have a larger field of view on 15x than on 22x for spotting your own shots.
On the other hand, if you are going to be dialing your drops into the scope, then you can use whatever power is appropriate for the situation, as you will be using the center crosshair for all your shooting.
I personally use the 3.5x15x50 with NP-R1 reticle on my 2 long range rifles. I dial the windage adjustment and use the reticle for elevation adjustments. It works fine on my 338 Edge and my 338 Allen Mag. Even with The Allen Mag, my 3.5x15x50 is all the scope I need for big game (Elk, Deer etc).
Good luck,
AJ