You won't always have a say in the match ups, but when you do you should definitely have a discussion about who you think would be the best person to send in to battle. So far I haven't seen a match up where one of the players couldn't beat the other though. My advice to you guys is make sure you stock up on as many items as possible and do as many practice battles to figure out the best strategy for your characters (every hero has already had at least one practice battle so far, so you guys are a little behind).