OK, so now we have some time to create a bit of a fantasy. Let's go back in time to the '70's. Now, each of us is in charge of a record company and it's up to us to decide which of the major quad systems for vinyl to go with. Would you choose QS, SQ, or CD-4? And why would you do so?

Since I brought up the topic, I'll start. Actually, my avatar speaks for me. I would choose QS. To begin with, it offers what I think is better stereo imaging
for two-channel playback. Rear information tends to sound wider in stage than SQ, which places it between the two stereo speakers in a basically mono display. A CD-4 record played in stereo merely folds the rear channels forward, putting the
rear channel sounds into their respective front speakers. While some may feel that means better compatibility with stereo, I like the idea of the matrix enhancing the stereo presentation. Second, the QS system was a regular matrix, meaning that separation was the same all the way around. With no logic, there was still more there than SQ. SQ emphasized left to right separation, and rendering only about 3 dB of center front to center back separation. This is why some form of logic enhancement was crucial for SQ. Sansui's introduction of Vario-Matrix logic rendered QS virtually discrete under ideal conditions. Although hose condiions were rarely ideal in practice, the process still rendered a better effect than the logic used by the SQ decoders of the day, pre-Tate.
The thrd reason is that, like SQ, no special stylus or cartridge were needed to play the record in quad. And playing the record in stereo would not affect the ability to play it in quad later on.