I honestly think there are a lot of variables to consider.
1. How long are you going and how many people?
2. What time of year?
3. What is your budget?
I have stayed onsite when I was a 20 something single with friends and offsite last 2 times....
Onsite was great when I was in my 20's. Sleeping wasn't as much a top priority and we were only in our rooms for a few hours each night...we stayed at CBR. Also, we didn't have a car so using onsite transportation was fine with us...no need to worry about driving home after trying to complete a drinks of the world showcase
Having stayed offsite a little older (30's) and wiser (I swear being a parent changes everything), I just can't imagine spending 7 days in a "hotel" room with 3 other people, even if they are family especially if it was in the summer. I just need a little more room to move and spread out. We are also planning on eating at our condo, so that's a factor. Eating out 7 days in a row for all my meals is a little too much for me....I'd probably relive my college freshman 15
Also, depending on where you look sometimes you can get great deals offsite for very nice resorts. Having said that you do have to include in cost of parking each day...so include that in your pricing. Staying offsite I would never recommend using their shuttle services, I have heard horror stories at even some of the nicest places (we're talking 1 1/2 hour waits).
I'm sure I will get backlash from the onsite faithful and I will say, for shorter stays (4 days) or if you really want to experience the entire Disney experience then onsite may be the way to go for you. Soak it all in and enjoy the extra benefits of EMH...
Since we are staying offsite for a week long vacation this time, I'm hoping to be able to do a return trip during the holidays and stay at a deluxe for a long weekend trip for DS and DB and DSIL.
Honestly it's like figuring out a complex science equation...plug in all the variables and hopefully you'll come out with what works best for you.