It depends mostly on which ninjas you have already and what kind of team you can build to match certain main character. If you take a look at posts in Strategy section- there's many suggestions of which main goes well together with this team or another and in which situations. You c*so post screenshots of ninjas you have, summons etc and I'm sure people here will help you to choose best strategy.
Also, as mentioned above- you can switch main again after 24h, so don't worry- change mains, try out which goes best with your style and have fun :D