Join Date: Apr 2005
Location: SW Washington, USA
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9 and 10-speed use narrower components and are possibly not as rugged. But they're bling, so the new bikes use 'em. I wouldn't think "chain stretch" is any worse, but certainly the gears would wear faster; all around, 9-speed uses closer tolerances which would have a greater chance to go haywire in tough conditions.
7 and 8-speed are cheaper, still easy to get.
I tour on 6 and 7-speed. Tight gear ranges and single-step shifting aren't that important for touring; racers need 'em to keep up with the wheel in front of them. On tour, you just downshift to the best gear and enjoy the view. You want a good range, but the most important gears are the highest and lowest.