I've killed 34 antelope, and in my experience approximately 90 percent of the time I shoot prone. I believe I can shoot better at 400 yards prone than I can at 200 yards sitting, but this is a test anyone can do.
I've used bipods by Harris that can be used both prone and sitting, and recently have been using a Snipe-Pod. The Snipe-Pod is much lighter than the Harris, and can go down lower, but is not quite as stable. It can also be used for sitting, but switching from a prone to sitting configuration takes just a bit longer than the Harris.