I use the 98-8 inch, because it allows me to see the dips in the slate much better...instead of bridging them with a longer level. The longer the level..the more it creates a bridge which will not allow you to see the dips in the slate as well. The 98-6 is tooooo short thereby to sensitive to dips and grind marks. I've tried all the starrett levels and finally settled on the 8 inch as the best for me.
Glen