As far as customizing goes, Gentoo is the extreme. You get full customizability in every way: kernel, software, optimizations, everything. If you want a KDE Desktop, so be it. If you only need a console, well, then you’re practically done after the installation. The one - and only one, as far as I’m concerned - negative side Gentoo can have is compilation times; it took me one whole day to compile KDE, and I have a 3GHz CPU. It doesn’t have to be a problem, though; just put the compilations on before you go to bed.
Ubuntu is the complete opposite of Gentoo. After installation, you’ve got a nice (some may say bloaty) GNOME setup (or XFCE/KDE if you chose X/Kubuntu) in front of you. You have much less (believe me) control over your system, especially if you’re new to linux in general.
I use Gentoo, but there are more options for customizability. If you want the power and knowledge over your system like Gentoo gives you, but don’t want to spend half your time waiting for packages to compile, Arch Linux is a very nice hybrid between the two. You have the same power, but with packages; I’d say it’s the best of the two worlds. The only backside of Arch I’ve found as of yet is its lack of packages; they have the most common ones (and uncommon ones, like SLiM), but as it is a fairly new distro (but a quickly growing one), you can’t except that much on the package side.