I am getting a new wheelset for racing so I am looking also for a new cassette, and I currently run a 9spd drive train. It seems to me that it is rare for anyone to run 9spd any more, is there a reason for this, should I go with 10spd?

9 speed is not "rare" but if you are building a new bike, you should go with 10 speed as 9 speed parts will be phased out over time.

You can still purchase 9 spd parts. But like what Android said it will be phased out eventually and harder and harder to find parts.

Buying a new set of wheels and upgrading to 10 spd could be a lot of $$$. I would just get a 9spd cassette and be done with it. Unless ofcourse you have a valid reason other than lusting over new stuff (like me) such as damaged parts or the like.

Most people, ie people who have not bought bikes recently still use 9 spd. There is nothing outdated about it at all. You can't just change your cassette and have a 10 spd bike. Well you could... but you could still only use 9 of the 10 speeds.

The only reason that I went to 10 speed, is that my DA 9 speed stuff was just about at the end of it's useful life span, and some nice lady decided to squish me with her car and damage all of my 9 speed stuff. Her insurance company was nice enough to pop for new DA 10 speed stuff (with a little arm twisting)

Definately go with 10 speed. You won't regret it. Having that extra gear either on one of the ends or in the middle strengthens your arsenal.

Going 10spd (assuming Shimano 105) will cost you a minimum of $400 over the cost of the wheels (a whole 105 group w/brifters, brakes, crank, BB, F&R derailleurs, cassette and chain can be had for $515). If you just want a new 9spd cassette and chain with the new wheels it could be as little as $65. I don't know your budget or your taste for parts (Campy vs. Shimano, and which level) but unless the rest of your drivetrain is on it's last legs I'd be tempted to stay with 9spd. Now, if $$$ don't make you wince then $1,150 for a Dura Ace 10spd group could fix you right up! Plus your wheels of choice.

In the end it usually comes down to bang for the buck.

Stay with 9-speed until your shifters wear out, they're the most expensive part of the upgrade. You can always upgrade to a 10-speed cassette at a later date, using the same hub.

Al