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
Neither is better. They both support the same tools and libraries. They are both linux. Anything and everything you can do on one you can do on the other.
Which os is better for development : Debian or Ubuntu?