We do both. We do chairwork per some of the stuff we heard at one of Nolan Ryan's speeches, we do shadow/mirror drills and we throw working on form from flat ground. We always get in mound work. We have 3 indoor mounds going and I move between the 3. We have half, 3/4 and full pen workouts.
When we say form from flat, we are talking about 30 feet from the other player working on various grips such as the curve. We are using 2 balls taped together, spinner disk, highlighted balls...
Whether ground or mound, they should always be preceeded by a good stretching program. We include band work. Then followed by a good stretching program. In either case, you also have to get some type of leg work in. Some believe in sprints while others believe in distance but that's another topic.