Some review said the KLF-20 had better bass definition than the KLF-30, and something for certain is that bass players like 10" woofers in their cabinets for a reason. The punch of the 10" is used in most all bass cabinet makers when they aren't using the common 15". You almost never find a 12". If you do it's something old and didn't last long in production. Some bass players use a cabinet with eight 10" woofers only.
I did a dirty comparison of one KLF-30 beside one Chorus II (both stock with obvious age), I like the KLF-30. The bass definition stood out to me. The horn is positioned a little too high as others have said. The KLF-20 is a shorter cabinet, I believe and if so it's probably about the right height.
But my room is also small. With upgrades to the Chorus II x-over and tweeters of both I might change my mind, obviously. Comping some 20s against 30s would be nice to do.