TBH it doesn't matter which girl you vote for. Aslong as it isn't Orchid, Cactace or Camellia.

If you really like the design of a 6/5* and want a new version of that aswell, go vote for that one. No matter the outcome, we are getting more 5 and 6* girls added to the total pool.

Having a 2* upgraded to a 6* adds just as much as a 6* getting a second 6* version. In the end the total ammount of new girls and thus to total new power we can obtain through sheer luck is the same. Since you are still going to have to pull the new girls from the gacha anyways.

Rule 3 should be: Vote for the girl you want to see more of. If its a 2* go for her, if its a 6* go for her. It doesn't matter.

So go vote for whatever girl you like, aslong as it isn't Orchid, Cactace or Camellia, since you are getting those even if they lose.