Should I level an Exeggutor or not?

I powered up one 86.7% ZH/P till 2278cp and other 100% ZH/SB untill 2596cp. Both are solid attackers and usefull.

Keep them both, #1 will destroy Vaporens well over 2000 cp just as he is.
Take #2 up to level 30 and leave him in a gym.

If you're only gonna do one, do number 1. Mine has the same move set and destroys everything in a gym, and dual psychic moves makes him perfect for a defender. There's no typing in pogo that resists psychic that's good enough to be effective.

If you want an attacker, then #1, if you want a defender then #2. Confusion / Seed Bomb is the best defensive moveset in terms of pure damage (some people prefer Psychic though for defense because many will attack with types that resist grass anyway).

The #2 is relatively better for defense than #1 is for offense, but it's still great even with Psychic.

If you dont have many candies/stardust, I would wait. Both are good but not optimal.

Zen/psy is versatile but not as usefull as Zen/SB fro offense and not a s good as confuion/Psy for defense.

2 is good but Confusion/Psy is better and you can find better IV also. (even if 87 is already good).

If I had to choose one, i would power n°2.