Choose a Mudkip

Please explain why you are so hung up on the 15 attack IV. Is there some particular important breakpoint that you won't ever be able to reach with a 13 or 14 att IV?

As to your question of which of these Mudkips to power up, it takes 91K stardust and 88 candies to go from L10 to L28, so that is how much you would save powering up the second one. Is it worth that much candy and dust to you to have the handful of extra IV points that you will never actually notice, except possibly having to power up one more level to reach the same quick move attack breakpoint?

I'd go with the level 28.

Swampert is decent, but there are much better pokemon for the roles it can perform, and several more close peers, some of which you probably already have. The level 28 will be semi-usable with zero/minor start dust investment whilst the same cannot be said for the level 10.

Swampert is a lot of fun. WG/S is an agile move set, and it has a lot of good resistances (just don't go mowing your lawn with it and you'll be fine).

That said, the second one. A million times over.

As others have pointed out, Swampert isn't particularly meta right now. So there are two options.

A) You want to use swampert in gyms. Go with the lvl 28 one, there's no point in maxing one out for minor performance differences that would only pay off at level 39 or whatever.

B) You don't want to use Swampert in battle. If you're a hardcore perfectionist collectionist, evolve the 14/14/15 one and sink as much dust as you feel like into it. If you just want a strong specimen of each fully evolved poke, evolve the 28 one and maybe push it to 30.

If you'll be getting a slow but steady amount of mudkip candy over time, my personal choice in these situations is to evolve the big one first, and leave the other as a future project should the candy and dust present itself--by that time you may have caught a better one.

:(