and Pushpak Bhatt Christian Boitet MG Abbas Malik

So for machine-translation purposes, this article suggests that written Hindi and Urdu are 97.5% the same (once you transliterate from Perso-Arabic to Devanagari or vv, obviously). So argue they're different if you want, but don't pretend that's anything but a social choice. Also, a good minisummary of what differences there are. Proceedings of the 22nd International Conference on Computational Linguistics (Coling 2008).