Vaporeons aqua tail and hydro pump are almost equal, difference is so small that it wont matter.
Aqua Tail is easier to use
- you can spam it more often, less dmg wasted if next aqua tail kills the enemy.
- Quicker animation time helps you to dodge.
In defense i would go for aqua tail.
Thou hydro pump isnt bad charge move, its not hyper beam, sometimes it can actually hit.
(Hyper beam still way too easy to dodge, even if you are half a sleep....and monkey)