What difference does stitch width make? (3mm/4mm/1Bu/2Bu etc)

Stitch width is literally how tight the stitches are. The smaller the number the closer together the stitching which makes the armour tougher and more enduring.
It is often used as a quick measure of quality, with the assumption that smaller stitching = higher quality.
But really the best measure is the use of high quality materials like deer leather for example.
So whilst stitch width is important, material quality is more important!