For us, off-site is much cheaper. Last year we saved $2,500 and that includes our food and character meals.
*We always want a car. In our opinion, the Disney transportation takes longer and is uncomfortable. We get AAA parking, so our car is close. We're back at our hotel in 15 minutes. It took the same amount of time to drive back to the on-site resort.
*We won't do the dining plan again. We don't want that many sit down meals because it takes up so much of our time. We ended up with so many credits leftover. We always have a full kitchen. I buy groceries the first day (arrival day for us is always at the resort) that lasts us 8nts. We make ADRs for the restaurants we want and the character meals we want but the majority of meals are eaten back at the condo.
*We always want a suite. My ds5 has sensory issues that make it very important for us to spend downtime in the suite every day. Even during downtime, he needs room to move around and I need the space for my sanity. The cost of a villa on-site is just ridiculous compared to off-site. We stayed in a 1bedroom at OKW and it was beautiful but so expensive. We got a 2bedroom condo for a fraction of the price. This year, we're getting a 3bedroom condo with a private splash pool, again for a fraction of the cost of the on-site 1bedroom suite. The off-site condos are in beautiful resorts and have all the extras we want.
The only negative I was worried about was that off-site didn't have a "Disney" feel to it but I quickly realized that it's all about how you look at it. At OKW, we certainly didn't get a lot of "Disney" feel but at the condo last year, I brought posters and wall clings and Disney toys ect.....it was very "disney-fied"! This year, our condo has Disney themed bedrooms.
I think staying on property probably has a lot of pros for different people. For us, for what we want out of a vacation, off-site is the only way to go.
Have fun planning!