My arguments to prefer water pulse over aqua tail are as follows:
1. Less predictable. Often you want to use your charge attack right after their special attack. Even when aqua tail can be used two times in a row, this rarely happens during a fight causing you to take more damage by water pulse because you are not able to dodge when using an attack such as solar beam.
2. Animation. In my opinion the animation of water pulse is more similar to water gun than aqua tail. Making it more akward to dodge.
In short, I would give the following rankings to different move sets:
Water Gun + Water Pulse -> Defender
Water Gun + Aqua Tail -> All Rounder
Water Gun + Hydro Pump -> Attacker
Making any vaporeon valuable
