Which Exeggutor should I power up?

Exeggutor is a good generalist attacker (Arguably only inferior to Dragonite and Tyranitar), it's a good blissey killer if you don't have acces to a good machamp or heracross, but I honestly prefer to max the attacking advantages so I rarely use generalist.

Edit, if you dodge, EX is probably more nimble, if you don't, confusion will probably be better.

Exeggutor is an amazing attacker! Power up that C+SB one as much as you can and he'll never let you down. He's one of the best against Snorlax and Blissey, and can fry multiple Vapes in a row. With that moveset, he's also a top-tier defender if his CP is viable in your area.

C/SoB is a monster.
I have C/SoB Z/SoB and E/SoB, and C/SoB makes my line up nearly every time.

He will do very well against a big Blissey, and he will melt Vapes and Rhydons very fast.

There are very few defenders that will make me change out from my Eggs once they enter the fight...and they get into nearly every fight.

Power up that C/SoB for now, and do the E/SoB later...would be my choice.

The C/SoB one can be used on defense to trap glass cannon attackers. (in my personal experience, you would be forced to dodge some Confusions, meanwhile that when Overheat was used, it was "unable" to kill the Exeggutor completely, however you need to power it up to at least 2500–2600 CP)

On attack, Extrasensory is better for dodging while Confusion being outputting more damage.

I prefer ES on attack because it is easier to dodge, but the C one is still awesome with a better iv and can be used on defense in the future.

The resistance to Pyschic helps in many Blissey matchups.