I would say use 2.5:
there is one reason to support 2.5 through 2.6: if you need to be compatible with python specified on linux or Mac installation (I don't know which version of mac mac provides, but you understand the idea).
Of course, if you need a 2.6 function, please do it, but if it is not, why require 2.6? Remember that your application will be hosted somewhere where deployment restrictions will apply.
If you will distribute your application on open source, all the more.
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