Overall for just decorations alone for me is probably IoA only because of Suessland. Grinchmas musical is great too. However, Hogsmeade decorations and protection show are ok and nothing special imo.
But ultimately, I'd choose US for the most Christmas feel. The parade, the tribute store, the tree, and the mini shows that are slightly reworked for Xmas (Blues Brothers and Marilyn Monroe) are all great and automatically put us in the Christmas spirit a couple of weeks ago. I love Diagon Alley's Christmas decorations more than most, but you really don't get a good feel for them except at night. We skipped the Cinematic Water Show to be in Diagon Alley when it closed and its so peaceful and beautiful to be in there with the decorations almost to yourself (we literally were the last ones in Diagon Alley by the time we left). My theme park hating wife even said that was her favorite part of the entire day was being in there then and leisurely passing by the Christmas Tree on the way out to a empty quiet park.
US would be my choice if only one day, but IoA has its plusses too.