Hup, what cassette are you currently running?
Do you need a higher or lower gear, is there a big jump between two gears where a gear between them would keep you in the cadence range that you prefer.
i like a cassette with closely spaced gears, hence the 12-27 that we use. I would not go back to a 9 speed.
What can be gained? A lower low gear, a higher big gear or maybe better spacing. It all depends on what you want.
i have never broken a chain, 5,6,7,8 or 10 speed. Yes I have ridden them all except the 9 speed. As for cost, there may come a time when 9 speed will cost more than 10.