But with how rapidly PHP/Ruby/Python/Java/etc advance, I prefer an OS that mostly keeps up with their latest versions. And Ubuntu's PPA system makes that a lot easier if you can find a PPA you trust.
When I first started, my predecessor had been fighting with upgrading PHP so that we could run Drupal on CentOS... 'Course he was still on hardware blades, not VM's. So he couldn't easily switch OS's. I got to move everything to VM's, and that's when I switch everything over to Ubuntu. After figuring out that CentOS wasn't the direction I wanted to go.
If you're hosting your own web apps on commodity servers, more into VMs and containers, or you are more concerned with tracking current versions of languages, kernels, and other software than you are with long-term stability, then Ubuntu can work well.
The 2 year LTS release cycle combined with 5 year support means you can either stay in step lock with LTS or skip every other version and still have a year to upgrade, I retired my last 10.04 earlier this year, that thing ran in production for five years with less than 10 hours downtime :).
It's also the (ime) best supported server distro in terms of third party packages.
In addition the LTS versions work well with vagrant and spinning up a dev box from a bash script is trivial (it's a couple of hundred lines of bash script most of which is reusable across projects).
If we where a massive shop I'd probably look at CentOS but so far I've not had the need.