There is a reason for that trend - Ubuntu LTS on the server takes less time to set up and manage than Red Hat/CentOS. Red Hat of course now does 10-year support for its releases as well as excellent security out of the box (10-year support is especially appealing to me as a system admin) but the ease of Ubuntu LTS upgrades and more frequent releases (i.e. up-to-date software) combined with the admin friendliness make for a more compelling business case.
The problem with Ubuntu is that if you ever move from the cloud to dedicated hardware, you will be in for a world of pain. None of the Tier-1 hardware vendors officially support Ubuntu, which makes it very difficult to perform firmware upgrades without custom solutions, or obtain diagnostic/monitoring software. We couldn't even get Ubuntu 12.04 to work _at all_ on our new Dell 12th-generation (Ivy Bridge) servers for several months after release. EL 6, on the other hand, was totally supported.
I hope that the availability of official CentOS AMIs will incent startups to think ahead and use it instead of Ubuntu, otherwise they could become a victim of their own success.
Another thing that's great about Red Hat and its clones is that they guarantee backward compatibility across minor releases (e.g. 6.0 -> 6.3), and ensure that if you deploy a given release it will be identical across all nodes. Contrast with Ubuntu, where an installation of a given LTS release (with "-updates") could vary depending on the day you deploy it.
