I'd want to play the bad ending first. I think I'd rather experience the absolute worst-case scenario and see exactly how things fall apart before going back and getting the happy ending. Knowing what could have gone wrong would probably make the happy ending feel so much more rewarding.
There's also something about seeing the characters at their emotional lowest that I just can't resist. I want to see all the heartbreak, the complicated emotions, and the choices that lead them down that path—even if it completely destroys me in the process. Then, when I finally get the happy ending, I can just sit there and think, "Okay… in this timeline, they actually got to be happy. We did it." And honestly, that feeling is worth all the emotional damage.