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
java and python would most likely run the same on both.
With Ubuntu you get additional space of support and active community, and perhaps larger user base. 
So if and when you face a particular problem, chances are with Ubuntu, the solution will appear faster. 
(although, whatever works on this should work on the other as well in theory)
Which os is better for development : Debian or Ubuntu?