I've used both and there are advantages and disadvantages to each. The single flopper does fly noticeably faster, penatrates better with less band power, and CAN be more accurate. I've also found it to be quicker and easier to remove from a fish. The disadvantages are that the flopper must be "tuned" properly, ie lay flat on the shaft in flight, open easily, and stay open. If not, the shaft can be inaccurate, and if the barb either doesn't open or doesn't stay open after hitting a fish, results in a loss.
The threaded twin barb will generally hold fish better, (assuming comparable wing span, as shown in your photo), opens more reliably, and can be replaced if trashed, without having to replace an entire shaft. It has the added benifit of allowing interchaneable tips for different fishing conditions. Twin barbs also require a bit more than half the distance on the back side of a fish to toggle. This is important when shooting fish up against rocks. The downside is that it really slows down a shaft, and punches a good sized hole, which is a bummer on smaller fish. These have been my experiences, and even though I hunt fish around rocks, I've pretty much gone with the single floppers for everything.
Ron.