on my true SS machines, with correct tension, the stitches have less diagonal to them. The length can be adjusted, and-the feed dogs feed straight so that you don't have any slight meandering. On my Morse multi-stitch machines, there is more of a diagonal to the stitch. And due to the larger openings in the throat plate and foot, it is easier to get a slight wavering of the stitch.
I am talking nuances here- but since I am currently piecing a 100% silk quilt that WANTS to wiggle, I will be quilting it on my 15-30.