Quick comments on the sheath & holster.
The stitching on the holster (second piece done) seems more consistent than on the sheath. Not sure if that's the case or just the way the pics look. If you don't use an marking wheel or spacing fork to position your stitches, using such a tool will help keep your stitches consistent in size and spacing.
The molding on the sheath leather looks fine to my eye. The single thing that is most noticible on the sheath can be fixed. That is the inconsistent spacing between the stitch line and the outer edge of the leather. A very careful trimming of the excess leather outside the stitch line could clear up the inconsistent spacing. If you use a stitch gouge during the construction of a leather piece, that offset distance from the edge can more easily be controlled.
Using a stitch gouge will do three things for you:
1. help keep your stitches in line
2. keep your stitches set in a consistent distance from the edge of the leather
3. recess the stitches slightly below the surface of the leather, which both keeps them away from abrasion and creates a shadow line in which the stitches lay that helps your sheath look more finely finished.
There are stitch gouges on the market that have an adjustable fence to keep the same stitch line offset spacing all along the edge. Otherwise, some makers just use their finger running along the edge of the leather as they grip the gouge to set the stitches in from the edge a bit.
I note that your rivets on the holster are not placed in the stitch line, either somewhere in the middle or especially at the ends & corners where the stitch line changes direction. Putting rivets at the ends or corners of the stitch line is something that I see professional leather sheathmakers designing into their rivet setting. I believe it's done so that the rivet (which is the stronger mechanical holding device) will take the brunt of a force trying to "peel" the leather layers apart, instead of the stitching (weaker clamping device) or the leather itself getting torn as the force exceeds the holding limit of the stitching or leather material. Alternatively, rivets may be used to resist some of the abrasion and wear at the tip of the sheath.
Fit and finish can make a world of difference in the overall impression of a piece. The mouth of the holster being left as a trimmed edge jumps out at me. A folded, rolled, burnished, or buffed edge there would IMHO look more professional, especially since the leather looks like it is pretty lightweight for a holster.
The later piece (holster) appearing to be an improvement over the first (sheath) would line up with what the leather workers in the crowd say: that practice makes perfect, so practice, practice, practice, then practice some more. The way it will usually work is that over time the overall quality and individual techniques get honed to higher and higher levels. I realize these are your first pieces, so I'm sure subsequent pieces will just keep getting better and better. The fact that there is a discernable improvement between your first and second piece shows that you are observant enough to improve at a quick pace.
My advice would be to spend time looking critically and minutely at the sheaths made by professionals that really catch your eye. When you spot a sheath that stops you dead in your tracks, take the time to reverse-engineer it in your mind. Try to figure out the methods and sequence the maker used to create it. Look real closely at how the sheath relates to and enhances the knife it houses. Finally, check out the DETAILS, DETAILS, DETAILS of the sheath. Really pay close attention to the "not so obvious" design elements that the maker is using to make their statement, because often those are the elements that are saying the most for him or her. So often it is the details - how many, which ones, where placed, how they flow together and back & forth - that will raise a sheath, holster, or other leatherwork (or any design) out of the ordinary into the exemplary.
Finally, as you've done here, ask around on the forums. The leatherworkers and sheathmakers have IME proven themselves to be a friendly and forthcoming crowd. Just some of the nicest folks you'd want to meet.
It may be informative to check out some of the many tutorials that Dan Gray has up on his webpage:
http://www.knivesby.com/knifemaking.html
Bruce Evans' leather sheath making tutorial, one of many ways of making a sheath, including use of a shopmade adjustable stitch gouge:
http://www.homestead.com/beknivessite2/pouchsheath.html
A glossary of leatherworking tools & terms that may come in handy:
http://www.bowstock.co.uk/tools.html
Besides the excellent tutorials and info here on CKD, there is also info to be gleaned from Bladeforums.com and Knifeforums.com, as well as the tutorial pages that some makers have on their websites.
Thank you for sharing your work with us and I wish you much joy in your leatherworking.