Matthew has a valid point. The economy of a stitching machine is not practical unless you make a lot of knives.....and sheaths.
If you are going to stitch by hand, the way to make it look and go better is in the layout. Purchase and/or make the tools and supplies needed to to a professional quality job. That will include a stitching pony, stitch wheels, groovers, edge slickers, edge dressing, etc. Make some stitching "drills" from old bike spokes. Drill every hole marked by the stitch wheel from both sides. Use a double needle system, and pull each stitch tight. Roll the stitching with an over-stitch wheel to set them tight. These and many other techniques are covered in the many tutorials, books, and DVDs available on sheath making and leather work. When you have learned to do all this, and the only thing you need is faster sheath production, then a stitching machine may be cost effective.
As for getting one for the "ease" of stitching, they are somewhat easier ( no pulling needles), but have their own issues to deal with. Any purchaser who knows his stuff will be able to tell a hand stitched sheath from a machine stitched one.....and will go in favor of the hand stitched one most every time.
All that said, I use the Tippman Boss. If set up right ,it makes a decent sheath. I am not making most sheaths for decorative purposes, just as containers for my knives. When I have a special knife,or ornate sheath, I make the sheath by hand.
Stacy