There are several "scale" couplers that have been used on S scale models:
Kadee O scale couplers have the advantage of being able to couple to themselves, Kadee S scale couplers, and AF compatible couplers. They have been used as a "more prototypical" coupler on some cars where most of the layout remains AF compatible couplers. Otherwise, they are considerably oversized for S scale. They allow automatic coupling and automatic or manual uncoupling by magnets, and they support "delayed uncoupling", but only with themselves. When used with AF compatible couplers, they only reliably couple or uncouple manually.
Kadee S scale couplers (802/808) are the most popular "scale" couplers in S scale at this time. Depending on which dimension you check, and which coupler your prototype car used, they are typically somewhat oversized for S. They allow automatic coupling and automatic or manual uncoupling by magnets. They support "delayed uncoupling". They have considerable "slack" in them, which is very noticeable when there are several cars in a train. 802 are black, 808 are rust colored. They come unassembled. Coupler boxes are included.
SHS 01295 couplers look and operate similar to the Kadee 808. The knuckle is slightly smaller, thus closer to scale. They allow automatic coupling and automatic or manual uncoupling by magnets. They do not reliably support "delayed uncoupling". There is much less slack in the couplers as compared to Kadee 802/808 (a plus in the eyes of many modelers). They have a smaller coupler box, thus will sometimes fit where there is not enough room for an 802. They are only supplied rust colored. They came pre-assembled (a BIG help)! The knuckle is .045" lower in relation to the mounting surface of the coupler than is the Kadee 802/808. This makes it ideal for the SHS open top hoppers and ore cars and SW-1/SW-9 locomotives. Other applications may require a bolster shim.
Sergent Engineering Couplers were designed for the best possible scale appearance while still allowing operation. They have to be uncoupled manually by using a small magnet above the coupler, then pulling the cars apart. They will not couple unless the knuckle is open. Sergent couplers come unassembled. Coupler boxes are not included, but a good article on building inexpensive coupler boxes for the Sergent couplers can be found in the S/Sn3 Modeling Guide, Volume 8/Issue 5, pages 19-21. Sergent couplers are still relatively rare in S scale.
Like everything else in life, there are tradeoffs! Most modelers will probably find the Kadee 802/808 or the SHS 01295 the best choice. These can be used interchangeably with the following provisos:
My approach has been to stock both 808s and 01295s, and to use the one that fits best in a specific application.
As usual, all comments/criticisms/corrections or additions are welcome!
Part 3: Step-by-step instructions