I decided against Sergeant couplers after investigation (no actual use) - but my reasons may not apply to you. Almost all of the "pros" turned into "cons" upon examination of my situation.
1) Scale appearance - this turned into a con for me because the Sergeant couplers are scale size for modern equipment (as is the Kadee #58 and other "scale size" couplers. My prototype era (1900) used a coupler 3/4 the size of today's. In 1900, some narrow gauge used the same size as the standard gauge (3/4 of today's) and some used 3/4 of the standard gauge (1/2 of today's). In HO scale, the closest to scale size knuckle couplers for my era is the N MicroTrains line. However, the gathering range (vertical alignment and horizontal swing) of the N couplers may be too small to be practical in HO/HOn3.
2) Operated by magnetic wand. Great if you can reach in to uncouple at all spots. Not so great if you want uncouple 2 or 3 tracks back in a reasonably spaced yard or need to reach around delicate structures/scenery to uncouple. I prefer to use some kind of uncoupling ramp for these situations.
3) Knuckle has to be open to couple, just like the prototype. Kadees couple automatically anytime they are lined up. Again, the benefits depend on how you operate, and how difficult it is to wave the wand to open the knuckle.
4) Does not appear to have a centering mechanism. Just as in #3, you must ensure coupler is open and centered for coupling, same as the prototype.
5) No fake "air hose" needed. A real plus for Sergent couplers. The same can be done for Kadees if you use bamboo skewers to uncouple.
6) Not compatible with other knuckle couplers including Kadees.
From what I can see, these are perfect if you model post WW2 or later, and use or plan to use bamboo skewers or similar uncoupling devices. I am very glad that we have these choices available to us. Try a few if you are at all unsure.
my thoughts, your choices