I know everyone will chime in and say it shouldn't make a difference, but I've been running a 10 tooth for the past eight years or so and I can't stand running the 9 tooth. It puts way too much pressure on the chain (being that the rear cog pulls so hard on the rear wheel) and it's impossible to get the chain alignment right without a half link, also rendering the whole rear setup weaker.
9 tooth + half link= going over handle bars.
I love 10-27 gearing and feel much more comfortable knowing that I'm not putting 500-plus pounds of pressure on the smallest point of my rear cog every time I pedal uphill.