I believe if you're able to use one conditioning device, it would be a jenny over a mill. That being said, many keeps have the mill as the main part of their keep. So I don't necessarily agree that a jenny keep beats a mill one "no matter what". When I used the jenny, I never used a counterweight as the type of strength training I wanted to do was more intense than simply a counterweight.

While I agree with the jist of the posts, I'll only say there have been many Grand Champions and DOYs that were worked on mills. Could they have been in better shape? Maybe, maybe not. Either way, I don't anyone can discount a keep done on a mill by an individual that knows specifically how to get the most from his dog using that particular keep.