Best of luck with the rehab. I would lean more toward the swim tank than anything. Mostly because the dog will not be carrying all his weight in the water and a sudden change in direction will not force him to carry all his weight and forward/lateral motion on that one leg. Rehab in a dog is very tough because very few dogs can pace themselves thru it. Swimming would be best and although I am not a big fan of the emill for conditioning it would be better than hand walking for rehab, as it can be better controlled. On a lead, especially a lead over six feet, if he sees something he wants he will bolt to it, change in direction, sudden burst, and all the rehab in the world is out the window.
Best of luck. EWO