Wow they are sold out, I didn't realize they went that fast. Unless you enjoy pain I would go with 21a as it will probably have easier access. I haven't been there much but I live near Elk City spent alot of time in 16, 17 and 20. It's some real steep country. Chances are that there won't be near as many roads as there are around Elk City. If your the "do anything" type go to 20 probably less pressure. But if your not physically prepared for that go to 21a. Thats my best guess.