The left picture is stitched in Aurifil 50 weight cotton thread. The right is stitched in 50 weight Tire silk thread. The Aurifil is cotton, so it has a flat appearance. The Tire silk is very shiny because it's silk. You can see from the pictures above they are both about the same thickness.
One thing I have definitely noticed is that when stitched in a high-contrast thread every little error shows. Good to know! Usually when I use contrasting thread it's only slightly different in color so any imperfections don't show much.