Neither John nor Julian, it's really about - Paul! I really reckon it's about him getting to that stage in his life of having to decide which girl he would choose. He may have started to write about Hey Jules, and maybe got the rhythm/pattern of words from humming and singing that line, but I think at a subconscious level it's all about himself. I mean, it has to be? But yet again it's one of those classic Paulie vague one meaning fits all sizes type of song. What does everyone else think?
(LOVE the flow chart by the way! So funny!)