I will still be taking my dig out fir walks, runs and sprint work, I will be using the mills mainly for when I can't take them out because of the weather. I will use the carpetmill for his sprinting sessions, I will use the other mill (e-mill or slatmill) for his walks but also for his running sessions and thats why I dont know which one to get for this. I have read that a carpetmill should not be used for long periods of time, this is why I will use it for his sprinting. And as you said a slatmill is for developing wind which from what I see and read is the one thats good for long running sessions, yet as you also said the e-mill is good for controling the dog's pace. So this is why im undecided on whether a e-mill or a slatmill for his running session.

Jon