Kabbalistically speaking, a round challah speaks of the continuity of the life
process; quite similar in meaning to the wedding ring. The braided, on the other
hand speaks of the inter-twining of all of creation.
I would profess that, for the most part, it is a matter of custom stemming out
of the various lands from which we migrated. Some preferred braided while others
chose round. In my own household, while I was growing up, I never saw a round
challah. Braided was the lechem of choice. Either, in my opinion, is equally "shabbisdik".
We say "haMotzi Lechem min haAwretz"....we bless not so much the bread but
rather the Source, Hashem, who placed the wheat into the earth for us to take it
out of the earth.
Next Shabbos, be creative.........buy one of each and dip them into loads of