It's a dynamic representation. I thought at first, that Aragorn is "the Man," if you will. But the problem with Aragorn is that he raised as an elf, and his identity as a Ranger removes him from the royalty that rules the whole race of men. Which means that he isn't really ready to come to terms with his fate. (Him being a little edgy about being the heir if Isildur and all that)
Boromir is the Man of the Moment, the moment being the Fellowship. He was the right representative for men at the time because he was ready to face the leadership supposedly passed down from his father. He had an agenda in mind, and it was all for the good of his people. Obviously, his agenda isn't the most ideal one, if not unrealistic (giving him the Ring wouldn't really turn out as he had in mind, would it?). Boromir, however new and refreshing his leadership will be, will pressumably under the power of the Ring.
Aragorn, on the other hand, although reluctant he seemed to be about taking on the role of King (in the films, at least), turned out to rule middle earth under a new paradigm sans the Ring, and evil altogether.
So in the story that circles around who represents Men involves a dynamic change: Boromir as the outgoing old, and Aragorn as the incoming new.
Congratulations on your 400th post,
rutslegolas !
I'm nearing my 500th.
(edit: at this post, for the record, my posts totaled at 496!)