Take a look at what you've got on your bike, and choose a tool to match. You won't need a Torx bit if you don't have disc brakes, for example. Or, if you have Mavic wheels with their special spoke nipples, not many multitools have a compatible spoke wrench -- but, thankfully, a few do.

I picked out a particular multitool for one of my bikes, actually. A friend was in a shop and texted me to ask which Specialized multitool I wanted at less than retail. I poked around the site and chose one with shims for separating disc brake pads, which I might need for the hydraulic discs on my mountain bike.