Slapper Bars

In answer to the OP about them pointing down, better slapper bars come with a U Bolt that goes over the top of the front segment of the spring, and keeps bars from diving during hard stops. On really violent applications, they keep the front segment from going S shape when torque is released, as in manual trans gear changes.
The reason that the pinion snubber became "the" traction device used in performance mopars is the primary axle, the 8.75 has a sheet steel housing. By limiting axle wrap from the center, it takes all the torsional load off the housing. You can literally dead hook 1500 horsepower in a 3000 pound car and not distort the stock housing, if you are using a properly set up snubber. Early Funny Car guys did it every day.
Once you go to any sort of outboard mounted traction device, the housing becomes a weak link and thats where back bracing comes in.
As for effective traction control..absolute toss up between the slappers and the snubber.