I agree with the previous post it depends to large extent what you want from the server by way of services. Both though are extremely stable and both Debian or Ubuntu will give you very little trouble provided you know what you are doing. If your applications and or services are going to be bleeding edge then perhaps Ubuntu is what you want. You need to remember though that Debian can still have bleeding edge applications and services used on them but you will need to read the documentation for things like dependencies so you can check they will work. In doing so though upgrades and security updates become then an issue.

This is more about preference than anything.

Ubuntu is more bleeding-edge and has newer package versions but Debian is more stable.