Dell (via USC) unpacks specific drivers for both RedHat and Suse to use after installation on its servers. As Ubuntu server isn't officially supported, this clearly isn't happening for Ubuntu.
Is Ubuntu server then running with native drivers? In that case, which drivers are native with Ubuntu, and aren't (e.g. are Dell replaced) with RedHat/Suse? Are dell optimized drivers available for Ubuntu somewhere? Can the ones for RedHat or Suse be used with Ubuntu?
Are there disadvantages of this lack of fine-tuned drivers, and if yes, what are they? Would they make the switch (and extra costs, as both RedHat and Suse Enterprise aren't cheap) away from the free Ubuntu server worthwhile?
Dell recommends Canonical's commercial support for Ubuntu server on its machines, is this recommendable? Do they offer fine-tuned Dell drivers to replace native drivers?
Just to quit with the usual marketing blah-blah like "is officially supported" I'd recommend to check the contents of Dell's special driver package. If it is an RPM it should be extractable. You can then check and compare the drivers. I don't think there are much differences.
I'm running mostly Debian Linux on my systems and Debian is usually kind of never officially supported, but there are exceptions (see comments).
But: It works. It works great. Why? Because the Linux kernel is just fine.
I'd recommend to use plain Ubuntu so you only have to care about Ubuntu's updates and changes.
The only driver that really matters on Dell PowerEdge servers is the raid-driver.
It depends on your specific controller which driver that is (e.g. megaraid_sas, mptsas, ...) and what minimum version is officially required.
Ask dell about the reqired version for your driver. Then the distribution should not matter at all.
I do not know the real reason for this minimum version requirement, since I got a couple of standard-distributions (CentOS 4, RedHat 3, SLES9, SLES10) that have or had older versions - but everything was working great. Anyway I used DKMS with the Dell-provided drivers for RedHat/SLES on those systems so Dell could not tell me the "not supported" stuff.
So my way of installing these drivers was to make a standard-install (I did not initially use the Dell-drivers) and then update the driver. The result was a working, supported system.
With Ubuntu you will never get there - the OS is not supported - but to be sure - take a closer look at the raid-drivers. Apart from that I would recommend Debian instead of Ubuntu for server usage. See this question and its answers...
If it's a new hardware, then you might miss a storage, network and other drivers which are newer and integrated by RedHat. If you run mission critical service, you shall use RedHat 6 and not Ubuntu, until they will certify their OS too, but how they eventually do this in future is an open question.