If I was going to stay in a regular hotel room (and that's what you get with allot of the DTD hotels, then I'd just assume to stay onsite at a value resort. If you were going to stay at a place that has suites, possibly to gain more room, etc. then I'd stay offsite. I think allot of things go into planning the vacation that will best suite your family's needs. One big thing being if you have a car or not. If you don't have a car, then staying onsite is a no brainer. If you have a car, then you have more options available to you.
Personally, we are not a value room type of family, but we are also not DTD hotel family either. We have stayed in the Disney vacation club rooms, Wyndham Bonnet Creek and even rented a pool house. For us, it's not just the value, but the space that offsite accommodations can give us.