Apparently Hoshido is really friendly towards grinding while Nohr is harder and doesn't allow for as much of it. This basically means that Hoshido is the only viable MP game unless you buy the other route as a Nohr player.
Thank you Ninty, your decision to split up a game that was intended to be whole for more monies is really shaping up to be a great decision.
EDIT: There might be something that caps MP units at level 20 and reverts stats back to what they were at level 20. While not as bad Hoshido still has the advantage since they can bring every unit to 20 while Nohr won't get that opportunity without something like FE: Awakening DLC that allows you to continuously grind on DLC maps.