Which Magikarp should I evolve?

If they are both the same Level, I'd go with the Attack 15. If they aren't the same level, I'd go with the higher level. The difference between these two is very minimal.

Equal IVs, so I'd go with the 15 attack. But both are good, and you won't be wrong to evolve either.

Over time you'll get many more 'Karp candy. I'm over 2200 now, without trying that hard, just not evolving every Karp I get.

Can't go too far wrong with either one. If there is a big difference in level, then I agree just go with the higher level.

If you are relatively new to Pokemon Go, be aware that step 6/8 for the Mew special research requires evolving a Magikarp.

https://www.reddit.com/r/TheSilphRoad/comments/88g3j4/how_to_catch_mew_step_by_step_credits/

I suggest the magikarp with 14/14/14 because it has better defense and stamina than the other magikarp.