I'd keep the KX, if I was in your position.
Really, the RM's only advantage is the bigger motor - the KX will have better suspension, brakes, ergonomics and will be easier to get bits for. By the year 2000, the 125s were quite tractable, and generally pretty good for trailriding.
The other thing to remember is that the early 90s was when the manufacturers started speccing the bikes for supercross, and lots of them had super fast steering and stiff/harsh suspension that made them awful in the bush. If my memory is still working, Suzukis of that era had suspension that worked pretty well for average riders, but were prone to headshake.
The only times I could imagine the older bike to be the better option is if you're seriously heavy and/or if you do a lot of gnarly hillclimbs out in the bush.