Powering Up Exeggutor?

I would take the first one.

If you are impatient and don't have enough candy, take the second one.

[Edit]
How did you manage to save so much stardust? Did you reach level 30 or more, never leveling any PKM? Why did not you start leveling after you found the first of your two top-Exeggutors? Very strange. For me, it seems as if you were patient like a bot....
[/Edit]

The candy would be my main concern. However, as attackers will perform more or less the same whether they have 75% IV or 91% IV, I'd probably take the 75% one to lvl 30 and then stop there. If he doesn't cut the cake as an attacker in that state, I doubt the 91% one would either.

If you need two strong grass attackers or plan on powering up way beyond lvl 30, you might then spend the rest of your candy and some dust into powering up the 91% one as well.

He has an awesome moveset and IV, but it's going to take loads of candy to power him up...

Did you catch it as an Exeggutor? If so, then good for you. If you believe you can get the candies then power it up - it's worthwhile since there's no guarantee you'll get the moveset again. If the candies aren't common though, go with the level 26 one.

Did you evolve a low CP exeggcute because of the IVs? That was your mistake then - but you're committed and power him up at this point. I mean, what else were you expecting? If you have candies to evolve a level 6 pokemon you have the candies to power it up.

There's not a big difference between 75% and 91% IV and the first exeggutor level is too low.

I'd power up the second

I'd power up the first, IVs will make a significant difference when PvP comes out, and ZH/SB Exegg is probably gonna be on your PvP team. Until then though IVs are overrated.

I'd take the second one, no question. There's no way that 16% difference in IV makes up for all of the extra stardust and candies you'd need to invest.