I don't have any scientific studies to base any of this on, but here is my $0.02 worth: I don't think aluminum vs. vinyl makes any significant difference. Was the aluminum darker than the vinyl? The color of the siding probably more difference than what it is made out of. Both of these sidings probably have an r factor of 1 or 2 and they both trap a certain amount of air behind them. Aluminum siding is so thin that it can't have any appreciable thermal mass that would store and then re-radiate any significant amount of heat. There are numerous other factors in the makeup of your house whos thermal impact far outweighs any marginal plus or minus of vinyl vs. aluminum. All other things being equal go with what you like and with what won't dent or have paint fade. If forced to choose between the two I will always pick rough-side-out cedar clapboards stained with Benj. Moore solid stain because I don't like alum. or vinyl.