My recommendation is to first develop your extension for Joomla! 3.x as 2.5 is "yesterday's technology," so either you are developing for "disappearing market" (i.e. J2.5) or an "emerging market" (i.e. J3.x) - personally, I'd go with the latter.
Then, once you've developed your extension for J3.x, if, and only if, you have a need to make the extension backwards compatible, then do so for J2.5.
If you need some guidelines on how develop extensions and make them b/c for J2.5, Timothy Plummer covers this in his book, Learning Joomla! 3 extension development. http://www.amazon.com/Learning-Joomla-Extension-Development-Edition-ebook/dp/B00E7NC9KU
Now - that's just my personal 'forward thinking' opinion on approach.
Hope that helps and best of luck!!!