Katri's Mama, I've only ever used the regular Thirsties covers, since the Duo covers are new and didn't exist when I bought the ones for my DD. I'd expect them to fit about the same way, except obviously the Duo ones are made to be expandable so that only 2 sizes will last from about birth to potty training. If I need to buy any new covers for this baby, I'm definitely going to try a few Duos. I'll be interested to hear if anyone here has actually used both.
Comparing the Thirsties and Bummis, I found Thirsties to be trimmer (less puffy/bulky), and less likely to leak because of the leg gusset that Bummis doesn't have, and as I said, the Thirsties velcro (and actually some of the stitching, too) lasted longer. Too bad, because Bummis has some cute patterns, but I'm sticking with Thirsties from now on. ETA: Hey, I just looked online, and Thirsties has some cute new patterns now, too! Score!
I never tried wool--the high price of the covers, plus the possible non-waterproofness, plus the special washing and lanolizing instructions for wool, etc. scared me off--so I can't compare to that.