There is no difference.. except that "Merry Christmas" is used.. and "Happy Christmas" is not.

Americans traditionally say "Merry Christmas" and people in England (maybe all of Great Britain or the UK, I am not sure) usually say "Happy Christmas." However, with the cultures of America and the UK constantly influencing each other, such as by popular television series and by travel in between the two areas, the expressions are not as limited to a geographic area or a particular English (AmE vs BrE) as much as they once were. The meaning is the same.