Upper deck is better and $30 extra is worth every penny! We were on NCL Pearl on the lowest deck. It happens to be where their gangway is. Port days were not fun getting in and out of our cabin or using the elevators/stairs because the people congregated there waiting to get off the ship.
Also consider what is near your cabin. We had one on Carnival next to a crew service area (blank space on the deckplan). The metal carts banged around all night and disrupted my sleep.
Now I look for cabins in between other cabins and on a deck between other passenger decks. I like to get cabins towards the aft of the ship. If you go far enough back, there's little foot traffic by the cabin. Some people worry about extra motion there. It never bothers me!