Are there any real differences between them?
I want to program in java and python. And of corse be a normal user: internet, etc
Which one will give me less headaches/more satisfaction ? 
And which is better for a server machine ?
Thank you
In Ubuntu it is a bit easier to install packages for Java development, but it doesn't really matter that much. Remember that Ubuntu is based on Debian, so it works the same. Ubuntu just adds more user-friendly GUI's.
Which os is better for development : Debian or Ubuntu?