Babies generally feel less comfortable around everybody. Nearly every snake I've ever had gets calmer as an adult no matter how much it's been handled. However YOU may feel more comfortable initially with a smaller snake.
But unless you know the background and source of the adult, I would still recommend a baby. You really wouldn't want to get stuck with a wild caught adult, but if the adult is a long term captive that's been somebodies pet for many years, it can be the better choice especially if you can get information from the previous owner about the snakes personality and idiosyncrasies.