Re: XS thirsties or Duo 1?
Get both! I had two xs and two size 1 and that lasted me for 5 months (I had a long skinny bean). The xs were what we ALWAYS reached for when he was tiny (up to about 10lbs probably), then the size 1 duos started working well, too.
We also has a size small thirsties, size small BSWW and BSB, they worked well for his night diapers which was a prefold with a hemp back up.