Hmmm...well, I voted elevated, but it's really more complicated than just one or the other.
Right now Fayde is on stall board only, with a couple hours of turnout in a dry paddock or worked in round pen or arena each day. Since she is in a confined space where she has to poo and pee in close proximity to where she eats, I use hay nets and buckets or corner feeders for grain. This minimizes the spreading around of hay looking for the "good" bits, and keeps her nose over the feeder to keep from spilling grain everywhere. As a result she doesn't have her nose and mouth down in the dirty bedding nearly as much. And yes, I pick her stall morning and night, but you know how it is...they make another pile as soon as you are out of sight!!
However, if she were in an outdoor 24/7 arrangement, I'm all for setting up many small ground level feeders to keep them moving around and feeding in a more natural manner.