Law enforcement, by far. We actually have something other than sirens in common with law enforcement. We have nothing in common with the fire service. Success as a law enforcement officer takes strong critical thinking skills, an attention to details, powers of observation, a knowledge of the techniques and importance of good documentation skills, and an understanding of sociology and social order. None of that is necessary to squirt water, and firemen are not hired for those qualities. So, if you're talking about the qualities that those people bring to the table that would suit them to the practice of medicine, there is simply no comparison between the two. All other factors being equal, I can turn an experienced cop into a much better medic than an experienced fireman, no doubt about it.
But yeah, EMS should not have anything to do with either service. And for the reasons that PapaBear [incorrectly] laid out, if any merging is going to be done, the fire department should be merged with EMS, and not vice versa. After all, we're the ones who are actually working for a living.