Now I don't know nearly enough about music theory to argue that minor 7th chords are more "important" than 6th chords. What I can say is that minor 7th chords appear much more frequently in the pop music I like to play. I have a lot of Hal Leonard-style EZ-play books, and 6th chords almost never appear in these.
Hi Ted,
Most of my old books are that type also, not that I have a problem reading music, but I love them because I can actually see the notes and chord symbols without squinting.
I have that problem too I fingered mode, I end up playing a 6th chord instead of the min 7th , now before learning something new, I just make sure I play the min7th in its root position.