It depends on who is sitting next to me. If it's a stranger, they get the armrest, if it's family, I take both. lol Actually I don't go to the movies very often.
The side my gf is on. We lean in, too.
It's been a long time, but I'd say whichever side I put my drink in. :)
As long as I get a cup holder, I don't really care. But being right handed, I normally use the right side if it's available.
I usually share one side of the armrest with my fiancé and let the stranger have the other side.