i would say go with the $50 board, not just because its way cheaper... but because it teaches you so much!
how to source parts, how to solder, how to program, how to debug problems you may have.
if you were going to buy a board for robotics i would rule out arduino as its troublesome as the lack of a power bus.
the roboduino is a good choice, i have one myself... the axon is great, its $100... but its worth it!
both of those batteries you listed will most likely damage your servos, normally the limits of servos is 4.8-7.2
a 6v battery is the best for this, some people have told me a 7.2v battery will reduce the servos life... i would go buy that 6v, and make sure it has decent mAh, at least 1800mAh(milli amp hours). this is talking about the batteries capacity, a 1000mAh battery can supply 1000mA for one hour, 500mA for two hours 2000mA for 1/2 an hour etc...
the $50 robot is aimed to be your first robot
and its cheap!
the $50 robot is a great way to give you a kick start with your knowledge in robotics, good luck RiceKing212.