Originally Posted by
jussik
also, why not buy both cogs and try each out? you won't even need to shorter/lenghten the chain when you swap between the cogs.
+1. Buy several cogs. Part of the fun of riding FG/SS is trying to choose the right gear for the ride. 42/17 and 42/18 both seem low to me for the road, but I don't know you or how steep your hills are. If you are a very casual rider, a difference of one tooth may not mean much, but if you really want to hammer along for a good distance, adding or subtracting a tooth can have a notable impact on your overall speed and/or level of exhaustion, and the only way you'll know which gear is best is to experiment with several.