I'm going to throw a curveball at you guys:
If ninjas prove to be more popular then they're going to lose, you know why? (more bad players in the mix)
Check out the results of the previous splatfests.
I don't think the popularity affects wins, but the demographic. Take Marshmallows vs Hot Dogs. Naturally, as kids love sugar, more young players decided to do Marshmallows, and they were much less skilled. Take Rock vs Pop as a second example: Pop is targeted to a younger generation while Rock has been around longer and is often targeted to older people. Not saying that younger players are always worse, but generally, people less than 10 are less apt to making strategic decisions and have less experience with videogames.
As with this splatfest, I have no idea which is more popular. This time, I'll just gauge the miiverse posts and choose which side seems to be more popular. I want to do Team Ninjas, but I rather not be stuffed in tons of matches against my own team.