The grocery service fruit can easily be half the price, but there are downsides with ripeness issues. WDW fruit seems to be fairly ripe, though I have not tasted the melon. The grocery service can be hit or miss. If you are staying for 10 days, you will want a selection of green and yellow bananas for instance.
Where are you going to store all your fruit too? A cut cantelope takes up a lot of fridge space.
I would recommend trying out some fruit from the grocery service, but still plan to buy some unless you have room to haul extra fruit back with you. That way if the grocery food is ripe upon arrival, you can consume that first and then buy fresh from WDW...and if the grocery food is not ripe yet you can buy from WDW first and eat the grocery fruit later.
Or you can stick to something safe like apples and clementines and not worry about ripeness.
I'm sure Mousekeeping would appreciate any uncut left-over fruit you might have. With their wages, they probably can't afford much fruit.