It would help to know what your shifters and crankset are. Any 9-speed Shimano rear derailleur will have the correct actuation ratio. As already said, if you have a double crankset a short cage derailleur work for your 12-25 cassette and would probably work for a 9-speed cassette up to 30 teeth maybe 32 if tweeked.
The difference in cage lengths is all about "chain wrap" (total capacity), not specific cog sizes. The total capacity formula is (big ring teeth - small ring teeth) + (big cog teeth - small cog teeth). The longer cage is to wrap up the chain slack when running on a very small chainring and a small cassette cog.