I have both the springer and walkydog. I much prefer the springer- I've had the dogs (I run two at a time) lunge after deer and squirrels on both of these, and I barely felt it with the springer.
My dogs are about 40 lbs; they are my jackrabbit coursing dogs and are sighthounds- they chase anything that moves. The park I run them in has lots of deer, so I needed something that could take the shock of 80 lbs of dog hitting it. The big spring on the springer, and the way it's shaped to keep the pull low, really helps a lot. But the bracket that holds it is fairly large, and there isn't enough room on the seatpost on my hybrid. So I used the springer on my folding bike, and the walkydog on my hybrid.
I also use the bungee coupler made by ruffwear, and one dog who likes to pull is on a harness, the other on a collar because she prefers to just trot alongside. If the dogs see me put either one on my bike, they run over and stand beside the bike.
vickie