I don't mean to be a show stopper here. It really depends on how comfortable you are in the OS you are using.
But from my perspective, it will always be nice to use an OS that is supported and a Linux OS that will not EOL at the end of 18 months or 2 years. Would this mean that you will be moving your collaboration suite in production every now and then because the OS is EOLd?
RHEL is definitely an ultimate alternative since it has a very strong support for it's OS. Ubuntu 6.06 aka Dapper Drake is another good option, as it has a server support for 5 years! (and that's a WOW for an open source).
Just a thought, not from a technical perspective but on a business perspective. You see, the CIO's job is to make the CEO sleep at night.
