Which Exeggutor to power up?

Both

Since the movesets and IVs are practically identical, I'd go with the zen/psychic one simply because it will require less stardust.

In the long run, confusion is stronger on defense and zen headbutt is stronger on offense. It is not a huge difference with Exeggutor, but there is a difference.

If you need more pokemon to put in gyms and have enough candy, I would probably favor powering up the one with confusion.

If you are rationing stardust and candy, then for now you might consider just saving so you can power up your two larger Exeggutors.

Zen/psych. I love the versatility that I can use mine for a great attacker and a great defender. though zen is slightly less optimal on defense, have you ever tried using confusion on offense? That's my one regret about my confu/psych slowbro. Only really good in a gym.

The 98% first, since it will require much less stardust/candy. The difference between both Zen Headbutt and Confusion on defense is minimal.