I considered dropping down to 20# tanks when I had to replace my 30's a year or so ago. Capacity would still be plenty for me, and it would make it easier to exchange instead of fill the tanks. In the end I decided to just buy new 30#ers and keep it the way it is. But 20 would work just fine too. I'm not sure who out there really NEEDS 30# tanks, it does cut down on the number of trips to the propane store, but even one tank full is enough for the longest of my personal cold weather trips. But I'm just a weekend warrior type.
Anyway, to your list, I'd put my money squarely on #6.