I have both, but if I had to choose between the two, I'd take the spinner and never look back. And if I lived in the North, as your net name suggests, where the birds have seen fewer of them than by the time they get to this end of the flyway, or in an area where dry field hunting for ducks held promise, that same decision would be even easier. If I weren't confident in my calling or blinding, easier still.
The spinner is a much more versatile (and mobile) tool. It will provide stronger initial attraction to more birds in many more settings than a jerk cord, and with a good remote setup, can be used just as a tease, like a jerk cord, for wary close birds. And it will do so while drawing attention away from your hide, when the movement of operating a jerk cord, per se, may actually be drawing attention to you.
I've eliminated the "jerk cord" shortcoming of movement at the blind by using a Mallard Machine hardwired to power and control from deep in my pit. The trolling motor foot driven MM is easily manipulated from a gentle dip to a rolling boil at the touch of the button, so it's a pretty sweet rig, particularly in a nasty spot like my floating marsh "Mudhole blind," where line operated decoys wouldn't just be one more hassle for the dog but might not want to move at all:
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The three coots on the middle left are my "jerk cord" decoys that can't be fouled by floating black dirt-bergs, other flotant or suspended humus below, like my line attempts have been. I think they're a slick, if expensive, rig, but as noted above, I can't credit them with nearly the usefulness of my spinner. Mostly just something to let me feel like I'm doing something about it when the water's flat calm - and I'll add that their use has generally (teal excluded) proven much easier over- than under-done. Aside from that, I've found my MM most useful, at least on calm days, to draw big ducks' attention from the blind while my less-than-stealthful hunter's ease (OK, duck) down.
While I find a spinner more useful and versatile, "The Robo bringeth, and the Robo sendeth away." A lot of the birds a spinner initially attracts are going to smell the rat and push off before coming in range. A good caller with a good hide can usually beat it for big ducks, at least by the time they're down here on the PhD end of the flyway and have been shot at by gosh knows how many spinners along the way. Might be others can turn more distant teal with a call than with a spinner, but I can't come close to matching the spinner in that situation, so my rule of thumb is: spinner on for teal and off as soon as big ducks are spotted, or not used at all if teal aren't about or wanted. (Definitely off when geese show.) If, however, my calling weren't turning the big duck trick or drawing attention from our hide was necessary, I'd want to see if what birds the spinner eventually pushed off weren't the lesser of evils. Also let the spinner run to up my guys' chances of gunning in my absence when I need to jump in the Go-Devil and chase long-flying cripples. And I'm not too proud to give mine a quick spin at the tails of birds that have shut off my call or set-up, like one might flip a flag (or toss his hat) for another crack at geese that have pushed off.
I'll add, on a personal note, that even after years of using it, I remain philosophically opposed to the use of any electronics in waterfowling and find spinners about as esthetically pleasing as a flashing neon sign would be in the field. I much prefer the idea of a hand operated motion devise. But I took the question to be one of effectiveness, and the above is my response to that.
