For a small company it doesn't really matter so much, probably whichever one the Unix sysadmin has more experience with. I have two VPSs which both run Debian.

For a small company it doesn't really matter so much, probably whichever one the Unix sysadmin has more experience with. I have two VPSs which both run Debian. For a large company with dozens to hundreds of servers, you generally want RHEL (or SLES). Usually you're running RHEL (or sometimes SLES) on Dell or HP servers. If your servers are having stability problems (or if it is a decided company policy), you want to keep the kernel and packages up to date with all the latest security and other patches for your OS version. You also want to keep the server firmware up to date. So if there's a crash you can tell Red Hat and HP, or Red Hat and Dell (or Suse and HP etc.) that you're running a fully patched server with updated firmware. Of course, you want the kernel to try to dump core before it goes down. As your servers go from dozens to hundreds to thousands, you will tend to see more and more crashes. At a company with a few servers, you might see a server crash every few weeks or months, at a company with thousands or tens of thousands of servers, you often see more than one server crash a day. You want to look for patterns - if, say, machines with a certain host bus adapter keep crashing, and the core dump seems to point to that, it's something you want to be aware of.