Thank you to everyone who left a comment and the few who e-mailed me directly. I never anticipated so many responses.
I suppose the question of "which way" was misleading because before posting the question on my blog the decision had been made to hang the painting as the artist had intended - vertically / portrait. It was very validating to see that nearly everyone who did comment or e-mail expressed that this was also how it should be displayed. Thank you again.