RD Tech Help

[irritainment]

Alright, so I've got a bent and broken RD that I need to replace. I've looked and got the model, Shimano 105 RD 5500. It's mounted on a 9 spd cassette that is 12/25. But as I'm shopping around online for a new one, I can't seem to find any. But there are a whole bunch of 5501s. Is there a difference between the two? Or will I be able to install a 5501 to replace a 5500? And are there any other models that are compatible, i.e. 5600,5700, etc. Also, should I get a GS or stick to the SS which I have now? Thanks a bunch guys!
If there is already a thread to this that I couldn't find, could you just link me over to there and I'll apologize for re posting similar problems!

[cxwrench]

if you're using a double crank you can use the SS. the GS is the long cage for triple. you can use 5500, 5600, 5501, 5700...doesn't matter at all.