There was a time that wheelie bars were simply anti roll over devices that kept cars off of their roofs. Essentially casters, they were located under the rear bumper and provided a positive stop that unloaded the tire and kept the car right side up. As time moved on, short, spring loaded bars came into vogue. The premise behind these was to stop excessive wheel stands, but progressively do so to stop the car from unloading the tires. They were typically equipped with a flat bottom bar that resembled a leaf spring and a spring on the top tube near the wheelie bar wheels that compressed as the bars were loaded. As ride heights and centers of gravity came down, tubular wheelie bars started to appear. Constructed out of chromoly tubing, they gave some flex as the car pitch rotated, but since these low cars did not throw the front wheels in the air as violently as the old straight axle variety, tire unloading was not an issue.
In between these various designs, there were those that pivoted off of the bumper area, using it as a fulcrum to mash the tire to the pavement. They were effective in keeping the tire applied, but if you remember Rick Dyer, Danny Scott and Leo Barnaby back in the day, they did little to keep the front end down. X's began to appear on the top of the bars to make it easier to access the housing with a jack, and wild looking single wheel bars started to pop up. (They are still in use today.)
The most common and useful wheelie bar is still the standard two-wheel version constructed out of chromoly tubing. Since the weight and center of gravity of modern Pro and sportsman cars is so much lower than the straight axle gasser of the sixties, they are the design of choice for everything from Outlaw 10.5 to Pro Stock. The reason for this is many fold; the rigidity they offer can correct starting line drift without utilizing pre-load (that can steer the car further down track and in the shut down area as the brakes/and or chute are applied). Length and/or wall thickness can be juggled to manage the hit they apply to the tires as they make contact; in classes that regulate bar length, you'll find bars that are of moderate diameter with modest wall thickness. Burly, stiff bars would blow the tires off on contact. As the bars get longer, the diameter and thickness may go up as the natural tendency of a longer bar is to bend more easily as it strikes the starting line. (This is why you see some 10.5 cars with 85” bars; many like the way they absorb shock.) We've had cars go 1.11 on 10.5's with 60” bars- its all a matter of choice and design. Did you ever notice how short Billy Glidden and Pat Musi's bars are? There is no ‘best' answer. Bumper/chassis clearance may decide length as much as anything. On all tubular designs the top bars absorb most of the brunt; their size and wall thickness are the most critical part of the equation. Once a wheelie bar reaches a certain length, it is necessary to ‘strap' the upper and lower bars together to add strength and to keep them from spreading at the hit. Knowing where, and with what material is a trial and error experience until you've done a few sets.
This doesn't mean that the sprung bar is a waste of time- but their design is somewhat misleading. Often, the springs on the upper tubes are not nearly strong enough to cushion anything, as the weight of a wheelstanding Super Stock/old school backhalf car overcomes them instantly. It's typically ‘visual engineering' at best. The flat bottom bar and its ability to flex helps these heavy cars stay hooked. Because of their high center of gravity, they come up and back very quickly, and the fact that they're so front heavy requires a higher nose height at launch for optimum traction. A chromoly tubular bar would have to be constructed too light to be practical in these applications.
Many funny cars, Pro Mods, Dragsters and Altereds have been showing up with single wheelie bars over the past couple of years. In these situations, they work great. They've also been tried on some slightly slower cars with little success. What separates the two is body roll- most dragsters, altereds, and funny cars are solid mount, so there is little or no movement of the housing up into the passenger wheel tub. Pro Mods and their stiff shocks and huge anti-roll bars also have little body roll, and do not need the adjustability of a two wheel bar that can adjust the handling of the car through the first 20 feet or so. Slower cars push the passenger tire into the tub quite a bit, and the stagger in the heights of the bar's wheels can correct minor starting line handling problems without pre-load adjustments, as previously stated.
In closing, it's important to state that there are no concrete answers to any of these options. Every car needs to be viewed as a total package, so consult with your local chassis professional before making any radical or unusual changes to a car that is close to right already. Safe racing- enjoy the summer.