The TwinSonic was available in 1968, the SD somewhere around 1976, so the TwinSonic was the first enclosed lightbar and had a big head start over the SD.
The TwinSonic had the aerodynamics of a brick and by the late 1970's aerodynamics became important as agencies were looking for better fuel mileage, thus, the square bars began to fall from popularity and the more aerodynamic bars like the Aerodynic gained in popularity. Basically, the SD arrived at the end of the square bar's era.
The SD draws more amps than a TwinSonic.
The SD did have a lot of lighting options. The twin rotators on each side with the outboard bulbs 180 degrees apart and the inboard 120 degrees apart gave it a unique flash pattern. SD's could also be equipped with oscillating, steady burn, or flashing lights instead of mirrors and the SD could be had in the very wide triple threat size (two speaker sections, three light sections).
Of course, TwinSonics had their options too. Cascade mirrors, X-mirrors, flasher light over top of a half-height cascade mirror, and the economy series which deleted the cascade mirrors or replaced them with either flashing PAR-36 lamps or a combination of PAR-36's and a V-mirror. Fed Sig did not have anything like PSE's Select-Alert or Night Probe.
Plus you have to remember that before the Twinsonic, everything was beacons... beacons on the Model 11 Twin Beacon Ray or individual beacons like the Model 17, Model 14, Model 184 etc. The Twinsonic was patented and was so unique that it carved a name for itself. The other companies' bars from Code 3, Dietz, Unity etc. were basically seen as wannabes although some of them had unique features too.
I remember NYC EMS using the Code 3 bars with all sealed beams, that's 8 sealed beams per bar, plus a minibar on the rear with another 4 sealed beams and of course all the other flashing lightheads and headlights and electrical draw on the ambulance. When they were idling the beams dimmed and the rotators slowed, you had to punch the gas to make them light up. :lol: