"And to you" or "you too"?
Solution 1:
They are both elliptical forms of the sentence "And Merry Christmas to you too". Either one is appropriate to use in informal situations. If you want to speak more formally, use the full sentence.
Solution 2:
"You too" sounds a little slovenly and maybe a touch insincere. "And to you" sounds slightly more sincere and thus more polite.
To my British ears anyway.