We have done both, and both have their benefits.Off site you usually have way more space for a lot less money. But for me, that is the only benefit. You have to rent a car if you don't have one (off site transportation is no where near as good as Disney's) and then you have to pay for parking. If you are a family who usually ends up splitting up so that some can go back to the resort to relax while others want to stay in the parks, now someone has to pay for a taxi to get around.
I like using DME to get from the airport to the resort. I like being in the Disney Bubble the whole time I am on vacation. I like being able to walk to a bus stop, and hop on a bus to the theme parks. I like Extra Magic Hours, and I like being able to have purchases delivered to my resort, so that I don't have to carry them around the parks.