If you get one, pick the Tiara. They eliminated the bump between the table and the machine. Using the sit-down machine, particularly with the extension at the side makes quilting much easier - or I should say it makes moving the quilt much easier. You can also add raised clamps to lift the quilt off the table and reduce drag.
Now that said, you will still have to practice. Just getting the sit-down machine won't "fix" the mistakes. Only practice will improve your skill. One thing I know from personal experience: We are often way too critical of our skills and we do see every off stitch because our noses are practically on the stitch. Walk away from your work and go back to look at if from a normal distance. You'll be amazed at how good it looks.