I just went through the same machinations, except 12-27 to 12-25. What you might not like are the three tooth jumps from 21->24->27. That's what drove me to the 12-25 and its 21->23->25. It was fine when I first started riding but now that I have a reasonable level of fitness and have learned to use cadence to my advantage the three tooth jumps really started getting in the way...one would be a little too hard, the next allow a little too much spin for where I wanted to be in a particular climb/situation. I just got the 12-25 (CS-5600, brand new, locally for $40 ) yesterday and put it on...haven't been out on it yet but on the trainer it was much better. I'm glad I did it. Eventually I'd like to get to the 12-23.