Documentation makes a big difference if your goal is to get a robot moving in the near future; I'm feeling the sting of that at the moment, having just bought a Traxster kit from a robotics shop in Los Alamitos that's going out of business. The Traxster is from RoboticsConnection, an outfit I already know is in the habit of pulling its online documentation the minute a new version of something is released. Sigh. Thank heavens for Google caching.
Even if you are building a robot to learn how to build robots, you want to bring it to life quickly; and nothing beats Lego Mindstorms for that - quickly putting a robot together and coding it in hours. Expensive but very fast; and if you can persuade National Instruments that you want to learn to develop Mindstorms software you get a student version of LabView to play with! Woohoo!
But sooner or later you want to roll your own (like zitron did!); and nothing out there now beats Arduino for simplicity and power. IMHO.