You live in Iowa and I used to go there a lot (Marshalltown) mostly in the spring/summer. I don't know what the winters are like there but if you're going to be out in below zero temperatures a Markins ballhead is likely to lockup on you after extended exposure, even if you take all the precautions against this in their instruction manual. The Markins are great ballheads but they can be a bit finicky sometimes.
A RRS BH-40 won't lockup and I understand the Photo Clams don't lockup either but I don't have personal experience of a PC ballhead in severe cold. I've begun to use an Acratech because I've experienced lockup here in New England and in the summer I'm on beaches and in the sea too. Maintenance is essentially just running the thing under a faucet.
I've had a Photo Clam Pro Gold II Easy QR for a few years and its has been great. I'd buy it again. I use it with a D3s and lenses up the 300/2.8 with 2x TC. I would have no hesitation in using it with a 500/4 either (probably with a Sidekick). My girlfriend has a Feisol CB-50D and again it has been great for the few years she's used it. Used with a 5D MKII, 70-200/2.8 IS II and 2x TC as well as the Sigma 150/2.8 macro, no complaints or issues.
