Before I really get into this: can G0-TO download himself into several things? If so, it's gonna be nearly impossible for Plagueis to win. Just saying.
OK so here goes. Strength in numbers here is pretty much unimportant, so no advantage here for G0-T0. This will be purely a battle of wits. If I'm either one of these guys then here's what I'm gonna do:
I'm gonna spend YEARS doing a bunch of stuff to throw the enemy off of my game. What I mean is that I'll spend time doing a bunch of bullcrap plans that end up getting seen by the enemy. I do this until the enemy thinks he knows me. Then I engage in legit plans to assassinate the enemy. This wins it here. I think it is gonna come down to how capable these opponents are of killing eachother. G0-T0, being a stealthy robot, is gonna be impossible to detect through the force, while Plagueis is gonna be nearly impossible to detect.
G0-T0 WINS!! Here's why: remember how Sidious was killed? He was backstabbed. He never saw it coming. This prooves that these Sith aren't invincible to deception. For this reason, G0-T0 wins. Thoughts?
I'm not sure about the downloading, but I would say no, he can only transfer into one other body. However, he could potencially do this to escape his ship, or even a planet to a different body stashed on another of his vessels.
Well, I agree with you EXCEPT strength in numbers. It does matter here, seeing as Plagueis has almost no numbers, while GO-TO has many, many assassins he can use to at least attempt to kill Plagueis with. Also, Plagueis's lack of numbers is what I think will drive this to a 1v1 duel, and a victory to GO-TO.
If the "greatest Sith that every lived" was picked up from behind and hurled over a railing, and wasn't able to see that betrayal coming, it's not a strech to say that a Sith without "Spidey-sense" will fall to the same fate from an invisible genius droid.