I like surprising our DD5. I think doing a little of both is the way to go. What we have done before is a week before our scheduled trip we would tell DD5 we are going to Disneyworld in 7 days. At the same time we would pull out a 7 day-countdown chart and post it on our closet door. Everyday she would put a sticker in the spot for that day of the week. On Saturday when it was time to leave she would put the final sticker on the chart and off we would go to the airport.
So in this case it is a little of both.