if you are using DME (disney magical express) to get back and forth to the airport, staying onsite is a must. Otherwise you need a rental car for the days offsite.
if you are not - your choice.
staying onsite means that Disney can move your luggage. but I generally have my own car - so I pack a small bag for my one nighters and leave the rest of the luggage in the car. So this benefit is not that great for me.
Do you want to visit the parks those days? would you like to be able to have the extra magical hours?
If yes then stay onsite.
If no - then I might be tempted to save money and stay offsite.