Ubuntu fixes the most important problem of Debian: its slow release process. Now, about half a year after the latest Debian release, I already move slowly from stable to testing and unstable. Not without reason, most often because new software versions fix bugs of their older versions in stable. More frequent operating system distribution releases can help to deal with that problem. I don’t know whether Ubuntu is more stable or less stable than Debian, I don’t use it regularly. But I know I’ve already installed Ubuntu instead of Debian more than once, because the more than two years old software distribution didn’t run on new hardware or didn’t provide software satisfying contemporary requirements. Ubuntu is natural choice then – it is very similar to Debian and it retains one of its big advantages providing completely Free Software distribution.
But I personally can’t move to Ubuntu. The major problem of Ubuntu I can see immediately is its limited scope. Some Debian packages are unavailable for Ubuntu and most Debian packages are not present in the officially maintained part of Ubuntu. This is very different from Debian, offering a huge amount of officially maintained packages. I install new pieces of software frequently and with Ubuntu it might be very time consuming and tedious.