I think by vertical bobbin is simply meant a front-loading bobbin (instead of a drop-in bobbin). Terminology issue.
Historically, I believe front-loading bobbins have always typically had a better reputation for good stitch quality and consistent stitch quality. As technology has developed, drop-in bobbins have become better. Not sure how one would compare the technologies these days, as a good quality machine with a drop-in bobbin may perform better than a low quality machine with a front-loading bobbin.
Last edited by Prism99; 10-18-2013 at 08:28 AM.