While that is certainly true, the main difference is that one could potentially acquire vastly greater numbers of gjallerhorns if they so desired with Xur. The availability of the gun shouldn't be considered, honestly, since both have a slighted form of RNG (only certain enemies have a chance to drop the Norfleet and Gjallerhorn). Your argument, while still valid, just appears to be imbalanced due to the differing natures and values of the game (such as Borderlands 2's sense of power creep with overpowered levels). It would make sense why one was better than the other

Then what about scaling? Even when scaled to Borderlands' level, it can't compare.