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
Ubuntu is the more user-friendly of the two (I think Ubuntu is actually one of the most newbie-friendly Linux distros), so if you are new to Linux, Ubuntu is the way to go.  Otherwise, the packages are mostly the same except for branding, so it's pretty much your choice.
Which os is better for development : Debian or Ubuntu?