Which Solar Beam to power up?

I would personally go with the Venusaur, all else being equal. Vine Whip is nice, dodging is very easy. And Venusaur has some nice bulk.

Vine Whip is better than Bullet Seed but have lower EPS. I'd personally power the Exeggutor BS/SoB, im looking for one like this to max out as a pure Grass beast. Exeggutor's Solar Beam is a shot, have more attack and hp than Venusaur.

The venusaur, for sure, if you like to dodge than I am even more sure. If you want to do an eggy, go for the extrasensory, is better against everything except rhydon (and others pokemon with double weakness to grass)

Extrasensory powers up solar beam very quickly. However, you may want pure grass moves, depending on how you plan to use it.

You migth want to run a few pokebattler simulations for all movesets to see which one fits the usage you have in mind better, make sure you select the type of dodgeing you use.

Bullet seed!!!

You will be using SolarBeam about every 5 seconds. Its Solarbeam spam!!

What are their current CPs? Try to place a comparable CP water pokemon in a gym and test each how each one plays. A lot of dodging play styles are personal preference.

Exeggutor has higher attack than Venusaur, but Venusaur with Vine Whip dodges easier. Between the two Exeggutors, Bullet Seed and Extra Sensory have the same time and close to the same damage (Bullet Seed Edges out against something that is weak to grass but is less against something like Snorlax or Blissey), but Bullet Seed does charge more energy.

If it were me, I'd go with the BS Eggy, but I already have 3 with ZH/SoB (plus a BS/SeB, and a C/P), so the double grass attacks appeal to me.

I love my Venusaurs, but they just don't have the firepower of Exeggutor.

All of them. They are all really good. If I could only do two, I'd do Extrasensory Eggy, because that guy also battles Snorlax/Blissey/etc. as a beast generalist where you are using SoB for the raw DPS, and Venusaur for pure grass, as long as you have the candies to max him. VW gives Venusaur the quickness that no Eggy has.