I prefer the K3 (electronic version) because it automatically adjusts, even mid-training, to keep the max effort around 75-80%. Your diaphragm gets strong, fast so if you don't adjust continuously soon there is not enough resistance. It's like lifting weights where a machine constantly increases the weight and manages it so you're always pushing yourself. You can do this with the manual ones, but I'm lazy and would rather pick up the device in the AM and PM, do it for 5-10 min, and be done not having to think about it. Keep in mind, though, the K3 is like $400...