I use the walking foot for straight lines in the ditch, & the free motion foot for curved or outline (and, of course echo, but that's a different topic, right?) ditch quilting. It is possible to use your free motion foot for ditch quilting, but more difficult to maintain straight lines and even stitches. If this is the foot you would prefer to use, maybe to avoid changing the foot real often, just practice & you'll get the hang of it.