70-71 e-bodies always have been and always will be more desirable than 72-74, much the same as Cuda's are fetching more money than Challengers even at the Hemi Convertible levels. The fact that the early years cars were available with the big blocks, and "most" consider them to have better stylng than the later years makes them more valuable. Yes as the years pass and fewer 70-71's are around the later cars may climb in value but I don't think the late small block ralleye's or Cuda's will ever reach the level of the early year cars because of the history. This is why given the opportunity most looking for a restorable car are still going after the 70-71's instead of the 72-74's. If you aren't doing a concourse resto, the price would be the same to bring either car to show quality but the 70-71 has enherintly more value.
all of this is my opinion and speculation based on what i have seen take place, but it doesn't matter what muscle era car you look at pre-72's almost always are more valuable than post 72's, be it a Camaro, Road Runner, Charger, Chevelle, or Mustang. 72-73 seems to be the breaking point because that's when the emissions and other regulations stopped much of the fun.
Brian