Hi there, gibsonfndr.
There are a few reasons for their relative weak showing on the 'Collector's Value' front. The looks' date=' obviously, as few people find the series two Firebirds prettier than the earlier ones; but there are more 'normal' reasons, too.
Quoting from Ian C. Bishop's "The Gibson Guitar from 1950" (1977 edition) :
" These {the series two style} guitars were probably even less of a success than their reverse-body ancestors had been and many big dealers still had unsold, brand new Firebirds of this type in stock right up to the early seventies, although production had ceased in 1967......{the second series version was} produced in much larger quantity and were considerably more unsaleable..."
The VII's (and V's) in both cases had the mini-humbuckers so should, in theory, sound similar.
The I's and III's of the '66 series, on the other hand, came with P-90's so will sound different.
The series one instruments had a through-neck body, which would likely help sustain over the second series which had the more usual set-neck.
I think I am correct in saying the nut-width of the second series was narrower than the '63 series so the 'feel' will be different (although I'm far from being an expert on Firebirds! I'm sure someone will correct me if I'm mistaken!).
The series one Firebird VII had an Ebony fingerboard (with large block inlays) and so will feel a bit different. All the series two guitars had Rosewood 'boards (and dot inlays).
Another thing that was regarded as a 'downgrade' for '66 was a change to fairly nasty slider-switches rather than the three-way toggle used on almost every other Gibson for p-up selection.