Unless you're paying Canonical for Ubuntu support, leaving Debian out as a choice doesn't make sense. Improvements to Debian (which include translations, packaging, fixing bugs, or even just filing bug reports) also improve Ubuntu. Ubuntu regularly rebases on Debian, at which point Debian's improvements (as well as so Debian's newly introduced bugs) find their way into Ubuntu. If you help squash those bugs in Debian, they won't be there affecting Ubuntu users. Also note that there are other commercial distributions basing their work on Debian, so improvements to Debian also improve those.
By all means, if you're looking for commercial support, go with Ubuntu (or RHEL).