I have a 12.9" Pro and use an Apple Smart Keyboard. It offers a great balance of usability and flexibility. It's a decent keyboard but it's also VERY easy to remove when I just want to use my iPad as a tablet. I tried a Logitech Create for a while but ended up returning it. While the Create is a better keyboard (backlit, better tactile feedback from the keys) it is awkward to use in tablet mode. It's very difficult to detach (the iPad physically slides into it) and folding the Create back requires that the metal iPad sit directly on the metal keyboard, which to me is worse than nails on a chalkboard.
If you see yourself wanting to sometimes use your iPad as a tablet without a physical keyboard go with the Smart Keyboard. If you see yourself always using a physical keyboard go with the Create. As a bonus, the Create provides all around protection (front, back, sides) but the Smart Keyboard will require that a separate, compatible case be purchased if you want rear case protection for your iPad.