Powering up Exeggutor for defense sadly got even less tempting with the introduction of dark type pokemons in Gen2 coming up. I have a 93% lvl20 Exeggutor with C/P and I can't decide whether to max it or not. They hit like trucks though, both on offense and defense.
Have you tried attacking with C/SoB? If you like any of them for attacking, I might have held out on powering up the ZH/SoB. If (when) your two lvl 31,5 exeggutors are too low cp to get a good position in gyms, they'll still be great for taking on Vaporeon as attackers.
Also, how many Exeggcute candies do you have currently?