This is just my opinion of course.
Gentoo is a source code based distribution. All code installed on a system is compiled and installed. (well mostly).
Gentoo has a tool called portage which enables a system admin to update packages very easily. However, its so easy that its easy to break the installation configuration.
Actually its not that easy, but because people don't read they end up replacing configuration files when they shouldn't and they lose they're configuration and whine about it on the forums. God knows I've hosed myself at least a dozen times before I finally learned all the necessary tools.
A gentoo install is very time consuming because all code is compiled. It helps to have a couple of machines in the houserunning distcc, but its still very time consuming.
Ubuntu is a binary package based linux. All libraries, kernels, and software are strictly controlled and compiled in advance to be compatible with each other. These packages are KNOWN STABLE and usually a few months or more out of date (depending on the code). If you need a cool new program you may or may not be able to find a binary to install. You can build from source on Ubuntu or any distro, but you have to configure that and learn how to do it by hand.
Ubuntu just works. Its clever at detecting hardware and it runs on just about anything.
Gentoo can run bleeding edge software, or leading edge software or you could let it get very very stale if you want once you get a system working. You can run almost anything and you can find an ebuild for almost anything (see
http://breakmygentoo.org) and you can completely hose your system by emerging something that isn't quite ready for general consumption.
Gentoo allows you to determine your compile options to tune everything for your processor. It eeks out a few percentage points in performance by doing that but again if you over tweak you can break your gentoo.
With Gentoo, building your own kernel is pretty easy and compiling it for your specific hardware is pretty easy as well. However I imagine that's pretty easy on Ubuntu (I did it with Mandrake years ago).
'emerge world' is why I run Gentoo, because once you know the necessary steps getting updated is pretty easy and painless.
Gentoo is more a nerdy nerd's distro. Its for hackers and tweakers who like total control of their system.
Ubuntu is for average users as well as nerds and is much easier to set up but, in my opinion, more difficult to maintain because you can't choose to install new software until someone compiles it for you (unless you want to install it yourself which can be a configuration nightmare).
My advice, since you are asking this question is to steer clear of Gentoo unless you are fully committed to learning the Gentoo method. It takes time, patience and a whole lot of reading.
However, once you learn, you never go back.
I run Gentoo at home, on my personal server, on a MythTV Box and on my work PC (without IS's blessing) and I adore it.
I have beryl at work and it rocks. I got it early on because someone made an ebuild... now its in portage.
Rock on dude!
Brian
Some day there will only be free software.