Oh goodness. You're opening a can of worms by asking people about strollers for older kids... From what I've seen, some people have REALLY strong opinions about this topic.
To answer your questions, I'd say that, given the ages of your kids, I'd do a Disney stroller based on the day and your kids' energy level/needs - perhaps you won't need one on Day 1 of your trip, when everyone has energy to spare, but as the vacation wears on and the kids are a bit footsore before the day even begins, rent one and see how it goes. (Some 6 or 8 year olds will refuse to get into a stroller. Others are perfectly happy to climb in and rest their aching feet. You know best what your kids will do!) If that's what it takes for you to have a sane, whining-free trip, go for it. A single is definitely easier to maneuver. Remember that the Disney strollers are all identical - probably wise to bring something bright (scarf, ribbon, etc) to tie to the handle so you can spot it among all the other Disney strollers in stroller parking.
Have a magical trip.