Even though it is an expensive colonial home, there is no place where I can put all the audio gear without Marital discord or childrens prying hands. The unfinished basement could be used but I hate artificial lighting an potential for dampness.
Not to be mean, but it looks like you've already answered your own question. If this is really the situation your better off with the attic than no tunes.
As for the better location, attic or basement? Throw out the numbers and it's likely Audiogon members will cast their vote.
Frankly, I would have no problem with artificial light in the basement if it had potential for superior sound. Besides, quartz halogen lighting can be beautiful if well designed and executed.
As for construction, no question the (concrete) floor of the basement would be superior to the attics 3/4 plywood. That is unless you plan to substantially reinforce it.
I have pier and beam and wound up with two layers of 5/8 tong and groove laminate ply, glued with liquid nails and power screwed to the lam beams. Then we poured 1.5 inches of concrete and then natural slate set in grout.
It is possible to get your attic that solid, but the sloping walls may be a problem as well.