Which 86% Magikarp to evolve?

I would go with attack, it will have higher CP and stack higher on gym.

Your magikarp #1 will give you a 2763cp lvl 30 Gyarados.

Your #2 will be a 2758cp lvl 30 Gyarados.

Make your choice ;-)

I would wait, remember it costs 400 candy for an evolve you want 90+ IV surely since it is so expensive.

First of all, they are not both 86%, the first one is 39/45 = 86.6, the second is 37/45=82.2

Technically the 4 STA will have a bigger impact on the dragonite's performance even if it yields lower CP, this is because Gyarados ATK is already pretty high (237) and it's STA in much lower (190) so 4 STA will have a bigger impact than the 2 ATK. The reason why the second one will have higher CP is ATK is weigthed more on the CP formula.

I'd go for the first one.

unless you are in a desert biome and will never see a better magikarp or you have gotten to 800 candy without a better one I would wait until you have a 90% + one.

but if you must go for number 1 as radiants reasoning is sound.

admittedly i had a collection of 96% ones from when they were in eggs funnily enough none came from eggs. I think they have skewed the Iv distribution closer to mean in an update at some stage as I am seeing far less 90% + mons and negligible amounts of 95% mons in the last couple of months.

If I remember correctly, at level 30, each 1 attack IV gives 11~12 CP, and each 1 def/hp IV gives 6~7 CP. All in the case of Gyarados. So the 122 CP one is more potential in terms of CP. The difference will be minor though. Also if you want to use it as an attacker, the 132 CP one is definitely better.

they are basically the same...the 15 atk one will have slightly higher cp