Both endings are very different, but it depends on the player.
Many players prefer the high honor endings due to the fact that Arthur has fully redeemed himself in order to save the Marston family and the rest of his fellow gang members from Dutch and Micah while others prefer the low honor endings due to its darker tone and the fact that Arthur dies at the hands of his rival: Micah without reaching his full redemption.
Personally, there are two endings that made the most sense for me (the high honor ending with Arthur going back for the money and wounding Micah and the low honor ending with Arthur helping John) due to the fact that Arthur would sacrifice himself and distract both Micah and the Pinkertons in order to save John.
Personally, helping John with low honor is the most realistic because there's a lot of tension between the two and the line that Arthur says to Micah before he was killed ("Damn us both!) put a lot of emotion into the ending. Considering the fact that I've written stories with a gritty and serious tone, I think that Arthur dying at the hands of Micah not only adds a more realistic and darker tone, but it puts more depth to the Epilogue missions later on in the game.
