It is an easy assumption to make, since the sergeant has two weapons and the private only has one. That's a complete upgrade. But after that, it is just a matter of weapon load out.

Remember that you have to upgrade the squad members as well. So just getting a team full of majors won't be as effective for you if you haven't upgraded them at all versus a squad full of sergeants if you have maxed them out.

So feel free to experiment. Remember to upgrade your squaddies and yourself and feel free to swap your squad around if the current set isn't doing you any good on a particular level.