It depends on the adults' vacation style as well as the teens' -AND- whether you will have a couple of cars or not. Chances are, the teens will want to sleep in a little longer, be out in the evenings. How would that affect the adults' plans, if they needed to meet up for transportation?
Once they are on WDW property, teens are not very likely to need to return to the hotel any more often than adults. They can independently get around WDW easily enough. Assuming they are responsible young people, you wouldn't have very different concerns than you would if staying onsite.
Either way, two-way radios or cell phones would be useful. We visit with teens and enjoy offsite visits without any trouble at all. Depends on the people in the group and how well they do with splitting up and reuniting.
Also, with hungry teens, having a kitchen to stock up for big, cheap breakfasts, snacks, soft drinks and making late-night, frozen pizzas and ice cream sundaes can save $$$. Stopping at less pricy offsite buffet meals can be a LOT cheaper than feeding 4 adults and 5 teens at Disney restaurant prices!
I believe the Swan charges a "resort fee" of $10/room/night at check-out, in addition to the nightly rate. Plus, if you use the valey parking, it's $10/night. Keep these costs in mind when planning your budget, as they can add up.
If you won't have cars, onsite is really preferable. HTH.