I have tons of experience with FreeBSD in an enterprise environment and can say hands down that pf is the best firewall money can't buy. That said, Red Hat has largely replaced FreeBSD for a lot of CIOs over the last 10 years. I really think startups should launch on CentOS instead of Ubuntu which seems to be the trend.
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.
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