Which Moltres to power up?

You will most likely not notice a difference when using it.
in theory:

87 IV with 11 attack or

more bulk and more damage delt until it faints, but
slower than

82 IV with 14 attack.

who is faster.

Your decision. :-)

Funny. I'm in almost the same situation, only with Articuno. Am debating whether to pick the 87% with 12 attack or the 82% with 14 attack.

I only caught one Moltres and it had perfect IVs with bad charge, but I don't know if I'll ever power it up.
I agree with Bruno, tho. Not much of a difference

https://www.reddit.com/r/TheSilphRoad/comments/6l2msh/raid_damagecalc_find_damage_cutoff_level/

You might want to check how the damage break points for 11 and 14 atk IV compare against some future raid bosses like Mewtwo and Celebi.

The 11 Atk IV will need to be powered 3 more times to reach the breakpoint of 14 Atk IV. But If you plan on powering it higher that that level anyway, then the lower Atk IV doesn't matter.