Ubuntu is mostly debian, and they even support a LTS (Long Term Support) version of their software. debian has more software packages (@18,000) than any other distro, so you would have any linux software you need right at your fingertips. Ubuntu comes with commercial support from Canonical if you should need that, but I think you will find lots of Ubuntuistas out there willing to help for free in the fora and such.
My personal preference is for debian, but if you are already familiar with Ubuntu, then use that for you web server since you will be able to use your already hard-won knowledege and Ubuntu performs admirably.
LTS is for long term support, meaning that they will provide help for it longer than the other releases. The most recent version, 8.04 is an LTS release. The previous LTS was 6.06.
My personal preference is for debian, but if you are already familiar with Ubuntu, then use that for you web server since you will be able to use your already hard-won knowledege and Ubuntu performs admirably.