I watched a video posted by Bernina on YouTube ,
http://m.youtube.com/watch?v=PjGEYnK3O_U and it looks like the dual feed system has a single, central top feed foot, whereas the walking foot has larger upper feeds that line up with the feed dogs along the side.
I would definitely see how it works without the walking foot befor I spent $185 on the walking foot. I think the walking foot is likely better, but for me, the dual feed is likely good enough.