If you're just watching them yourself, or with friends who have already seen all the films, it doesn't really matter which trilogy you watch first.

If you're showing them to someone else who has been generally unexposed, then it becomes an issue. Personally, I think the theatrical order of OT first, then PT makes more sense. "I am your father" is more of a surprise, and some of the smaller bits of humor in the PT have more resonance in hindsight than they do chronologically ("Anakin, some day you'll be the death of me.")

Another thing to consider is the focus of the story. OT is essentially Luke's story, with Vader being a supporting character that we learn more about when we see the PT later on. Watching the PT first, the saga now becomes Anakin's story, with Luke being a secondary player.