Carnahan has been around for a long time, and he knows his stuff from experience, but he's expensive and I wouldn't be surprised if that was because Dr. Henkel is SO much cheaper. That being said, Carnahan's facility is more professional and seemed to be kept the cleanest (though the Henkel's always disinfect). It's also easier to make a spay/neuter appointment with him because Henkel's schedule is usually already full.
That being said, Carnahan is also very liberal with anesthetic. I had two cats spayed at the same time. I took one to Henkel and one to Carnahan. The one I got back from Henkel was wide awake and active, while the other was groggy for nearly 4-5 hours after pick up. The same for a friends dog who was spayed. It's probably just hearsay, but I'm told it's because Carnahan spay/neuter price is based on the anesthetic he uses. Neither vet tests for heart worms or kidney health before prescribing medications or performing surgeries. These have to be asked for and most of the time they'll try to talk you out of it.