I'm not sure what you mean by the quality of the stitch. If you mean whether the stitches are all of even length, then you can do that much better when you have stitch regulation. Only the Bernina offers true stitch regulation on a DSM, and it is not perfect. The closest to perfection IMHO is the Lightning Stitch on the Innova longarm.
If you are talking about tension, that is something that can be adjusted on both DSM and longarm. If all the components of the machine are in good condition, you should be able to achieve really good tension on either one.
There is another possibility, and that is the "straightness" of the stitch. The old machines without zigzag capability make beautifully straight lines of stitching. Many DSM's that allow you to control the stitch width do not give stitches that look truly straight. Of course if you're quilting curves, you're not looking for a straight stitch.
Can you show an example of the type of stitching problem you're having? That would really help.