What kind of machine do you have? Viking has a new walking foot that has a snap in stitch in the ditch foot. It's pricey, but is on my wish list someday.
If I had to choose between the two, I'd pick the walking foot. I don't stitch in the ditch as much now that I have my new machine with decorative stitches. I am very fond of the serpentine stitch and it looks great! Very forgiving if you waver off the line. :)