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 (which you'll want for 3D graphics on an nVidia card). 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:
- Software packages I develop are guaranteed to work (with no extra dependency testing) for the rather large number of businesses and non-supergeek users who have chosen Ubuntu.
- The Ubuntu bug reporting/tracking system is far more user friendly than Debian's.
- I use an nVidia video card to play games.
A side benefit of choosing Ubuntu: A free multi-platform build farm and publishing system called Personal Package Archives. (Only to be used for freely redistributable software, of course.)