Both are fully supported so you should see good performance and reliability with each.
If it was my choice I would go with Debian. Debian servers are easy to manage - rarely go wrong, very few updates as a general rule (far more with Ubuntu).
Debian is less bulky than Ubuntu, Ubuntu is less stable.
Ubuntu has newer components which means newer updates for programs such as MySQL, Apache etc. Debian prefers older tried and tested versions of server components.
Think of Debian as that old dinosaur thats been around forever and is rock solid and think of Ubuntu as the new kid on the block more modern yet lacking in certain areas.