I like the hole that a French iron leaves, but I find them difficult to use. The bottom of the teeth are flat and lining up on the stitch line is not accurate.
The tips of the diamond chisels are pointed so placing them on the stitch line is easy and accurate, however, I am not a big fan of the hole they leave although I don't think it really matters since it will close up again after stitching.
I like Kevin Lee's black stitching chisels (Japanese Diamond Style) that provide the best of both. The tines are pointed and easy to line up and the hole is somewhere between the French and diamond styles (go to 6:30 in the accompanying LeatherToolz video and you can see the hole it leaves.
I have stitching chisels in 3.0, 3.38, and 3.85 mm and use them all depending on the project. For belts and holsters, I use the 3.85 mm.