I think it depends on the condition of the bodies. Go with the better. An advantage of the KM is it can take the M42, with an adaptor, and K mount. Advantage of the Spotmatic is it will probably outlast the KM because of the build, at a time when reliability was key, where robustness = reliability and, metal was king. I just bought an earlier S1a that does not even sport the meter, so no battery. The Spotmatic's meter is not mercury battery dependent as is so many older cameras so, that is not an issue.
If the bodies are close to being the same condition, then it comes down to body features. I've used M42 lenses on K mount cameras and I se no issue but I am used to using M42 lenses as my first slr was a Spotmatic. I bought it not long after they were coming back from overseas with a lot of returning service personnel. had a friend who bought a very complete kit, came home and needed the money to get married.