I also recommend doing multiple test rides. Just last weekend I tried out both a Giro 20 and 26 to use for commuting. I've ridden these bikes before on other test rides, but for some reason the Giro 20 felt more twitchy to me than it did before. However I do prefer being able to put my feet firmly down on the ground for stops and starts, so I'll probably lean towards the 20 as well.