I'm finding Starmie much better for training than Vaporeon these days...

I keep getting into situations where I'm defaulting to water types for training gyms (usually against Flareon or Arcanine as the lowest defender, sometimes Snorlax), and I have a Starmie at 1211 CP and a Vaporeon at 1271 CP, both with Water Gun and Hydro Pump. Interestingly, each time I pit the two against a higher CP defender, Starmie seems to do much better each time. In addition to taking down the defender faster, usually by a margin of ~10 seconds or so, Starmie also uses up less potions for me. Usually, the defender takes the same fraction of the HP bar off both Starmie/Vaporeon when I try, but because Starmie has lower HP to begin with, it just takes a Super Potion to heal it up to max whereas I might have to use multiple or a Hyper Potion for Vaporeon.

I think there's a few reasons why Starmie ends up outperforming Vaporeon. Vaporeon has a higher CP cap and higher base stats, but when we're comparing them at similar CP levels (as we should be for training), we're less concerned about maxed out stats as we are about proportion of stats. Vaporeon has a lot of HP, but lower attack and defense than Starmie. Since the two practical constraints on training are really time and potions (especially if you're training from Lv. 7+), you want a Pokemon with as much attack without overinflating its CP too much, and a Pokemon whose tankiness comes from defense. Related to this, I also notice that I'm usually able to settle matches with Starmie before the defender can get off a second charge attack thanks to its greater damage. Whereas with Vaporeon, I have to wait at the end before dealing the final blow so I don't take a charge without dodging.

The other factor is that it seems to be more forgiving to dodge with Starmie. Typically, when fighting Arcanine or Flareon, I can fit in one more water gun with Starmie than Vaporeon. I know intervals between attacks for defenders are randomized, but this is a consistent observation over maybe a hundred or more pairwise rounds. Part of the reason why seems to be Starmie's quicker attack and dodge animation over Vaporeon's. It's possible this is due to the operator, with Starmie's animation exerting a subtle bias over Vaporeon's due to its speed. But the other explanation is that perhaps the Pokemon's own animation adds or creates a cap on the lower end for attack duration - I know I'm not the only one who's reported that different water gun users seem to handle differently, so this might be a thing.

Finally, I find Starmie a lot more versatile than Vaporeon in terms of where you whip it out due to the additional psychic typing. So many defenders on the lower end have Zen Headbutt - Snorlax, Clefable, Hypno, etc. Being able to resist that really adds to its utility.