My M-9 OKC fits my 590A1 with the lug.
My OKC 3S does not fit because there is about a one-eight or three-sixteenths difference in length in where and how it secures to the gun.
Apart from that, just comparing the two in hand, I'd prefer the M-9 with the Bianchi military holster and wire cutter scabbard. It's a thick hardy blade with much gadgetry going on.
The other has a longer blade that can be sharpened pretty well, but I like the robustness and the saw cut better on the M-9.
If you like to entertain yourself with oil stones, the S3 give you more pleasure just like one of the other guys posted.
I happen to use the M-9 around the house, in the garage and in the shop. I take it hunting. The OKC S3 is pretty cool but sits in the drawer a lot.
I never looked into it, but I would expect a longer cap screw or cap nut or whatever they call it would be available and then the S3 might fit.