seems like you know your way around linux, so grab what you wish, I'd personally recommend NEITHER. go with ARCH.
Ubuntu is fantastic, i love it, and i use it as my main os on my laptops, perfection, the downside is that its i386...aka not optimized so speed lacks. on the upside the community is HUGE, support is HUGE, and packages are PLENTY. Most things are done automatically so you'll save lots of time over manual config / compile in gentoo.
Gentoo...you're running it, you know it can take ages to configure things properly, and if you need a program NOW, well, you cant have it now because some programs take up to a few hours to compile. Gentoo is sweet for the learning curve, it really is the best way to learn the guts of linux, and if correctly done its fast, so worht the price, the question is, are you willing to invest the time and the effort?
So why ARCH anyway?
I find it a happy medium between the 2 distros you listed. Its build in a Slackware-like style where configuration is (for the most part) manual, but much easier than gentoo and what have you. The package manager (pacman) also installs binaries which makes installations faster, and they are i686 + other tweaks ... ie.. Arch is tuned for speed while still be easy to install and maintain. You will need to invest a good day's worth of work to get arch running the way you like it, and there is no automagical program like Automatix available to get you there, but it works, and its fast. The wiki is far from complete, but very usefull still, also the forums and the irc channels are great to get some help.