Steve,
I use the stiffer belts while rough grinding and switch to the "J" flex , which are usually made in the finer grits anyway. This fits the natural progression of things. So, I always buy both. I start with 36 (stiff), then 60 (stiff), then 120 ( J flex), 220 J flex, etc. I usually stop grinding at 320 grit and start hand sanding.
Lin