I would think it would depend on where you're staying. Are there enough amenities at your resort to keep you busy a couple of days out of your trip? Some people spend a couple days visiting other resorts (AKL, BWV etc.) and enjoying their own that they've paid for. Also, will your first and last day be mainly used for travel? And lastly, will you be visiting any other local attractions or possibly a day at Disney Springs?
The cost difference isn't much as you add more days but multiplied times 10 people (if you're paying for everyone) can be costly.