Copied this from my answer to a similar question in a previous thread:
The advantage of 10-speed is smoother shifting, but not so much in the mechanical sense. Smoother shifting as you will have one fewer 2-tooth jump in gearing. Here is the “tooth-count" on the standard 11-23 cassette, 9-speed vs 10-speed
9-speed: 11-12-13-14-15-17-19-21-23
10-speed: 11,12,13,14,15,16,17,19,21,23
As you see, the 10-speed adds a single 16-tooth cog (a $15 part, ). With the 9-speed (on the 53-tooth chain ring), you shift from a gear ratio of 6.1 (17-tooth) directly to a gear ratio of 6.9 (15-tooth), whereas the 10-speed gives you an option of a 6.5 gear ratio on the 16-tooth.
More meaningfully, at 80 rpm on a 53-tooth this translates into speeds of 19.5, 20.7, and 22.1 mph for 17-, 16-, and 15-tooth cogs, respectively. On the 39-tooth chainring, these speeds are 14.3, 15.2, and 16.3, respectively). This may illustrate the extra effort you will have to put into that mid-gear shift. Personally, I'm rarely on the big chain ring so my shifting is centered around the smaller cogs, the 15-tooth is a climbing gear for me.
Edited by the bear 2005-04-22 12:15 PM