How many miles do you go in four months?

Nine speeds is a lot. I have a 9-speed Tiagra/105 drive train on my commuter, and a 10-speed Ultegra on my road bike. I don't notice a huge difference ... honestly, I'm a little more wary of cross-chaining on the road bike, so I probably have about the same thing. I wouldn't actually go and downgrade this on my road bike, but if the situation found me, I wouldn't lose sleep over it, either. With 15,000 miles on your shifters, the right/rear one is probably getting close to worn out.

Supposedly the thicker cogs and such mean a 9-speed system will generally last longer than a 10-speed one. I'm seeing the opposite between my two bikes, but I think a lot of it comes down to the FSA cranks on my commuter bike not being friendly to chains.