I don't know the origins of why different cultures say it differently, but I'm sure you could search for it.
One thing I can tell you with certainty: Xmas is not a word in the English language. I learned that by watching 'Almost Famous' :-P
I say Merry Christmas (erm... I think?), and I'm from Britain.
But... yeah... why is it that even the most un-weenis-like thread just has to turn all Baer-related...?
We usually say "merry christmas" along with "happy new year"
It dont quite sound right the other way around
I thought Americans say "happy holidays" anyway.