Firstly, while both paint jobs are crisp, K-Line colors are notably lighter. I have no reference for which deco is closer to prototype. Next, K-Line’s cab has occupants and window shades, and Railking doesn’t. Railking also has the red body stripe continue right through the cab, unappealing. K-Line has added on details like rungs and brake lines. RK detail is all molded in. K-Line has smoke, RK doesn’t. Both have dual single roller pickups, china drive, electrocouplers, and directional lighting. Neither have lighted number boards. K-Line’s model was produced by Lionel, with Railsounds & TMCC. Some of the K-Lionel RS3s DID have K-Line Cruise Control, but mine did not. RK has PS2 with all the usual features, including speed control. My K-Line was fitted with Cruise Commander M by Pat (Harmonyards). Both are great runners and pullers. RK’s FYS are just plain goofy, and, as this is a 2000 Vol 3 release, I believe DCS was still in it’s infancy. The K-Line model has generic early garbled crew talk, and generic Tower Com, and was released in 2009 by Lionel. K-Lionel’s RS3 came in many other flavors, (Circus, NYC, Western Maryland, LIRR, PRR, Southern Pacific, Reading, Cotton Belt), with RK’s initial release limited to EL, RI, and B&M.
In terms of looks, K-Line wins hands down. Sounds - Railsounds beats DCS sounds. Performance is a wash. Smoke works well on the K-Line version, and this iteration of the RK RS3 wasn’t offered with smoke.
Apparently, the molds for the K-Line version are no longer available to Lionel, so this more detailed RS3 will not be produced again. Lionel does produce a significantly less detailed RS3 in their Lionchief line.