I have stayed both onsite and offsite and find that you have to decide which of the pro's and con's of each is most important to you. Generally speaking the following will hold true most of the time:
$ - offsite, you will always get more bang for the buck offsite
space - offsite unless $ have no impact in your decision, then it's a toss up
transportation - with no car, definitely onsite, with a car, can be a toss up
meals - if
ddp is important to you, then onsite, if $ rule then offsite
touring hours - onsite, although many avoid emh, it is still a good benefit
independance - if your group may break up, then onsite
Some of the supposed benefits of staying onsite can be smoke and mirrors such as charging to your room or package delivery. There is no difference in charging with a room key and then a charge card vs. a charge card other than a couple of days before it appears on your statement. If shopping in the parks, you can always have your package picked up as you leave the park.