I recently did a comparison between Ubuntu 10.04 and RHEL 5.4 to determine what our standard server install should be. Just to be honest, windows server started out to be in the mix but was quickly disregarded as it really doesn't lend itself to scaling out in any reasonable manner. I set these up using the rackspace cloud which I have to say was a dream come true. I had both servers up and running j2ee application servers in less than 1 hour.
First off, I first used redhat back when it was shiny and new (94-95) and used it for quite some time afterward. I've also used a variety of other unix and/or linux systems (slackware, debian, ubuntu, suse, caldera, SCO, solaris, aix, hp-ux). That having been said, I am a linux USER, NOT a kernel hacker. So the inner workings are quite as important as much as getting a useful and reliable system together quickly.
What I discovered is that Redhat is trying to build a stable platform that doesn't change. They are committing the ultimate act of software suicide that IBM and Microsoft (and now even Apple) seem to have perfected in recent years. The fact is that Ubuntu has begun to figure out what makes things tick and they are iterating and evolving with the marketplace.
When I looked at the packages that were supported on the two platforms, everything on RHEL was at LEAST 3-4 years old and some of it was older. I understand from a business perspective, they think this makes sense because it costs money to innovate, but it is a serious problem. They are effectively ignoring 4 years of serious progress.
Ubuntu, on the other hand, was seriously up to date. Ubuntu 10.04 is current as of April and I know they will be release a new upgrade October. In addition, I have the flexibility to back/forward port new packages between releases if it becomes really necessary.
If you want outdated software that will work great as long as nothing in your business changes and you're OK being 4+ years behind everyone else, choose Redhat. If you want software that will evolve and change with your business over time, choose Ubuntu. If you want software that nobody will fire you for choosing, choose Microsoft (or IBM).