The Gold Eagle is the broadest retail benchmark because of recognition and market depth. The Buffalo is cleaner from a purity standpoint and sometimes tighter on premium.
If your goal is bullion exposure with maximum resale familiarity, the Eagle often wins. If your goal is .9999 exposure and a more direct melt comparison, the Buffalo is compelling.
Collectors should also consider grade availability and population trends, especially when buying slabbed examples where the premium spread can widen beyond the raw bullion market.