I used 8" for a long time, but injured myself. When I came back I "started" with 6" and never looked back. The 6" gives me a great workout...after all, you can add intensity into any workout if you want to. I enjoy it more because I can do tricky moves & higher intensity without constantly worrying that I will trip or fall. Everyone's different, of course. I'm in awe of people who use 10".
I have noticed on many of the newer step workouts that a lot of people are lowering their steps to 6".