i know the question was about market value, but it seems to me that gold vs. ss would also depend on the particular watch. some just look better, to me at least, in one metal or the other. i think a day-date looks great in white gold. submariner looks great in stainless.
i think it would also depend on the wearer. perhaps i make a virtue of necessity, but yellow gold is too flashy for my tastes. although, i would give serious through to the fifty fathoms in gold because the strong black character of that watch works real well with the gold.
if you go on the collectors market forum, though, you'll probably be able to form a quick impression of how well the vaues of various metals hold up.
chris