Howdy, Pards,
For most of my work I use a lockstitch, as I can pull it snugger. Depending on the job, I may use a sewing machine, especially for flat work (stitching belt loops to holsters, cartridge loops, etc.). However, for the main seams of holsters, and a lot of other stuff, I use one of two types of "needles". For most heavy work, where I can get at the back side, I use a Landis machine hook (a "needle" with one side of the eye open...they are made that way). You push the needle through the leather, either through a pre-punched/drilled hole or you CAN push the point through the leather itself. After pushing the machine hook through both pieces of leather, you loop the thread over the "hook" and pull the thread partially back through the leather, forming a loop. You unhook the "needle" from the thread. You then stick the free end of the "near-side" thread through the loop, and pull both ends tight, making sure the threads are evenly set in the middle of the two layers. Makes the stitching look like machine stitching.
For main seams on holsters, I punch the holes in each part separately. I may punch the holes using my sewing machine with the thread removed from the needle, or pre-drill the holes with a #56 drill bit after marking the leather with an overstitch wheel in a hand tool.
Sometimes, where I'm working in close quarters, I'll use a regular needle in a hand haft (Speedy Stitcher).
For light jobs I use #207 nylon thread. For sewing belt loops to holsters, etc., #277. These are the equivalent of military 3-cord and 4-cord respectively. For hand-sewing the welts on holsters, I use polyester "Coarse" thread from Stewart Manufacturing. That's similar to 4-cord.
Hope this is of interest.