The 20's are nice tanks and don't feel much bigger than the 13's. I think that the 20's are a great alternative for the 13. Catalina makes a great tank. The neck is short and beefy. The in water characteristics are optimal. The tanks will go from negative to neutral.
I commisioned Catalina to create this special size tank for the Rapid Diver system that I developed/invented Here is link if you are interested:
Rapid Diver Compact Scuba Systems
We have sold hundreds of these tanks over the past couple of years. Besides being used on the Rapids, the 20's are a being used as bail out for OC. I haven't heard of anyone getting one for their rebreather. I have some friends with Optima's and KISS rebreathers. I will get one of them to rig it up with the 20 to get some input.
We have the Aluminum 20 Cu Ft tanks in stock and are available directly at:
Accessories for the Rapid Diver Scuba System
Colors:
Gloss Black
Bare brushed finish (silver)
There is a 20 under the Rapid Diver Rebreather in the picture on the left.
If you are interested in trying out the 20 cu ft tanks, I have some at my dive operation at Weeki Wachee Springs
http://www.diveweekiwachee.com/