Have you thought about other options? A kit is an easy way to go for a first robot, and will definitely remove a lot of headaches. If this is your first option, you'd probably have the best luck with the second kit (the wheeled one). Robotics is difficult all on its own; no need to add the headache of walking if you don't have any experience with a general wheeled platform first. I only say this because you might end up getting frustrated with a walking robot, and robots in general - when it is really the issue of walking, and not robotics, that would make things difficult and frustrating.
Here's another low-cost option - if cost is a factor:http://www.budgetrobotics.com/item/ArdBot-Chassis-Kit-340
You'd have to supply other components, of course (so it may or may not end up being cheaper than your other kit options).
Another option to consider is repurposing an old radio-controlled toy car (something made by New Bright or Nikko) - in many thrift stores (my favorite is Goodwill) you can find perfectly serviceable R/C toy cars for $5 or 10.00 USD. All you have to do (once you know the motors, gearing, and steering mechanism are OK), is trace the connection from the motors (or other actuators) to where they connect on the PCB back to the control system (in most toys, its a single R/C chip). Most of the motors go thru some kind of discrete component h-bridge made from transistors or mosfets, and eventually end up at the IC. You have to also trace the power and ground connections to the h-bridge, then carefully remove everything not needed (R/C control IC and support components), leaving you with the h-bridges in place with power and ground, so that you can splice the Arduino in fairly easy.
Not for the faint of heart, but you'll definitely get a good education in the process, you'll have a cheap robot platform (in some cases, better than anything you could buy), and the satisfaction of knowing you did it all yourself...
Good luck - whatever route you take! You'll have an exciting and educational time no matter what, no doubt.