You made up your mind, so I'm not gonna go on about the four Jack's cats I run. Just ordered a fifth pair of Jack's tubes to build a research boat for channel mapping.
To get some exact values on tube diameter, length and draft, check out the Jack's website http://www.jpwinc.com/catdraft.html
for a table of the draft for each diameter and length. You can use it to figure out loads and drafts for tubes by other makers. If you care about handling, the max load waterline should be about 1/3 of the diameter (e.g.
for 24-inch tubes, an 8-inch draft).