Which os is better for development : Debian or Ubuntu?

Since Ubuntu is based on Debian, development is almost exactly the same for both. They're both quite suitable for server machines. The fundamental difference is that Debian follows a Free software ideology, while Ubuntu sacrifices that purity for practicality when no Free equivalent exists for important proprietary software.

If you choose Debian, you will have a choice of distribution series ("unstable" / "testing") that may get you newer releases of pre-packaged software a few months sooner than Ubuntu. Unless your development projects require bleeding-edge kernel or support libraries, this probably won't matter to you at all.

If you choose Ubuntu, certain proprietary software might be easier to install because it will be available through package repositories. For example, nVidia's proprietary video driver. That's not to say you can't make such things work on Debian; they will simply be easier on Ubuntu.

I personally choose Ubuntu, for these reasons:

• Ubuntu has a free multi-platform build farm and software hosting system called Personal Package Archives. (Only to be used for freely redistributable software, of course.)
• The Ubuntu bug reporting/tracking system is far more user friendly than Debian's.
• Software packages I develop are guaranteed to work (with no extra dependency testing) for Ubuntu users, of which there are many.

I'd seriously consider switching to Debian on my workstation if they offered a PPA equivalent. I don't use Ubuntu-centric stuff like Unity desktop anyway, and I no longer need nVidia graphics drivers (I finally got tired of their deeply broken OS support and switched to an AMD card). I already run Debian on my servers.

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.

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)

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.