It depends on what you are going to use it for! If I bought four plugs I might put Ubuntu on one, Gentoo or Slackware on another, Fedora on another, and OpenWRT on another! One for my collecting data for my weather station, one for a music server, one for doing some stuff for work, one for NAS.
Please VOTE: What is the best Linux distro for the SheevaPlug and why is it the best

1) Best size ?
2) Most features ?
3) Most reliable ?
4) Ease of installation ?
5) Other ?
Thanks Any information submitted here will be closely perused by me and others no doubt. I am looking forwarded to being educated.
Best
Gus