As an alternative to Virtualizing several Ubuntu Servers im looking into the possibility of using containers. But I am not sure if I should go for OpenVZ or LXC. Can anyone recommend one of them, or maybe list pros and cons?
Personally I find LXC is in rapid development and documentation lags.
I personally use openvz on "production servers" and do not think LXC is yet mature enough to use in "production".
I suggest you look at proxmox
Offers a Debian host, web interface, and KVM and openvz. They are planning to use LXC once they consider it stable.
From what I can tell LXC has some advantages like being available on the Linux kernel (so you don't need a special kernel to use it). From the other side, OpenVZ supports live migration.
You will have to check the capabilities of each tool and see which ones are most important to you.
You can find here a short review of the capabilities of each own.
Update for OpenVZ 4:
As of OpenVZ 4, you can run OpenVZ on a stock Linux 3.0 kernel. OpenVZ still benefits from it's own kernel; there are some OpenVZ kernel specific features.