Hi treetop,
I grew up fishing for salmon along coastal WA state and used/use downrigger often. I've used them on Powell, Mead and Mohave as well (particularly for the latter two, downriggers are more work than needed most times of the year). Not so for salmon, though.
I have found the the Scotty clips work best (for me). Be aware that they have two tension settings that can be increased or decreased by moving the head of the clip back and forth. I always fish mine on the stiffer of the two setting options. I sometimes troll six to twelve inch flashers followed by a squid-like lure and the clips handle the tension just fine; and that's a lot of tension from the flasher. I ensure I set my fishing line as far back into the opened clip (pre clamp down) as I can.
Scotty clips. (Length of connection line is a matter of preference).
Having said that, I do occasionally go through an unexplained period of three or four drops (up to 150' average) where I pop it off. I'm not sure why that happens but generally blame not loosening my reel's drag enough when putting down. I rarely pop free while trolling. Or at least if I do, there should be a salmon making it pop free.
I rarely pop off from my electric downrigger. I think its clutch and brake are smoother than the manual riggers. I fish Scottys on my boat but grew up fishing Penn. I've fished Canon's on friends boats. They all work fine.
I've used the Cannon clips and they are also fine, but feel as though they pop free a little more often. I do like that they have versions that allow for more control of the tension.
Growing up I used many different clip options. I think that the ones that they make today are better than ones used in the past. I love buying gear (says 'Captain Obvious') so in this scenario, I'd purchase a few of each kind and tested them out.
Hope this helps a bit. Ultimately, I think as with about anything, the more practice and refinement at something, the better it works. So... time to go fishing again!
Thanks as always to Wayne and all involved for such an awesome forum.
F/W Fishwhistle