Well the Mauser is a much more refined weapon, and of course has had more customs built on it then probably any rifle. Used all over the world as hunting rifle, in design.
But the Mosin is a great rifle in it's own right. You mentioned the floor plate letting in dirt, but I just went an looked at my M44 and you can remove the floorplate in 2 seconds and clean it out and put it back in in nothing flat. The Mosin is pretty well thought out and one thing that hasn't been mentioned is one of the most famous snipers that ever lived, killed over 500 of the enemy in 6 months time during the winter war between Finland and Russia. His favorite rifle was a reworked Mosin. He was Finish soldier and chose the MN over the Swedish 96 mauser for his work.
I would choose the Mauser, but the MN served it purpose well.