In almost all cases the ship is "berthed" on embarkation day. Meaning sold out, and passengers assigned cabins. It does happen sometimes but rarely. They even have a list of people waiting stand-by. Also, if were unsold cabins they would most likely be Inside.
The Inside cabins are the same as the Outside ones if is not Upper/Lowers ie Bunkbeds. If is making you worry and is less than $50 total I would consider it, if more than that is a waste in my opinion.
Best wishes for a wonderful cruise,