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
Both use Debian packages and Ubuntu is based on Debian but is more user friendly. Everything yo can do on one you can do on the other. I'd recommend Ubuntu if your new to linux on a Desktop. Though when it comes to servers I'd recommend Debian as it has less stuff "taken out" basically.
Which os is better for development : Debian or Ubuntu?