Re:
Well, technically the Lava Dome would be better as it sat a couple of rungs up the pecking order than the Fire Mountain. Although truth be told I doubt there's a great deal of difference in the frame material between the two frames.
I think perhaps the geometry might have changed slightly between '91 and' 95, I could be wrong there, but personally I always seem to get on better with the "later" 90's Kona's (I have a '94 Fire Mountain and a' 97 Lava Dome).
It is pretty subjective though, and I'd probably go with whichever fits you best frame size wise and whichever you like the colour of!