We currently have a LAMP Ubuntu Desktop 10.04 server with a RAID 5 configuration. We are at a point where I can upgrade that to an Ubuntu Server 12.04 edition (+gnome or xfce; for maintainability after I leave) or CentOS 6.3. I've searched the web and have seen conflicting reports of which I should go with. I would like it to be efficient should we suddenly get a bunch of traffic, yet not difficult to grasp for someone with primarily Ubuntu knowledge should something go wrong. I do not know whether there are any significant performance gains when choosing one or the other.

A package-manager is being used to update programs since I do not know if the maintainer will have knowledge/time for building from source.

Ubuntu 10.04 Server Edition will be fully maintained through April 2015, there is no rush to update:

https://wiki.ubuntu.com/Releases

You can easily update your 10.04 directly to 12.04 if you wish. However, if you do so, it is prudent to make a complete backup, test that your sites & apps & services will work correctly with the updated system, and (if your server is mission/business critical) thoroughly test the upgrades on your test machine before applying them to your production server.

CentOS is very nice too, I'm not saying anything bad about it, just that if you are currently happy with Ubuntu, there is no overwhelming reason to switch.

(ps I do not recommend Gnome/Xfce for a server; any sys admin who replaces you in the future will know command line, it is a basic linux admin 101 skill.)

Canonical recommends for users of Ubuntu 10.04 LTS not to upgrade until the first point release was made public.

Also, I can only second what snowpine posted, if your future admin can't do his job without a GUI you should consider to send him/her to a training course or choose a different, more knowledgeable, person for the job. Simple question: What would the future admin do if the server fails in a way that wouldn't allow him/her to start the GUI?

All the important info has been placed by guys above..
...
...
And as per my opinion...CentOS would suck less. Attachment 10095

As soon as you install a GUI on a ubuntu server release, it basicly becomes a desktop version with server stuff as well. So unless the desktop packages are removed, its only supported utill April 2013.

I probably wouldnt go building software from source with a server OS at all. IMO there is no real advantages, and several disadvantages.