Use JVN's design calculator to choose your ratios, and model the results;
http://www.chiefdelphi.com/media/papers/1469
Don't forget that under robot weight, don't just put 120. Add in the weight of the battery and bumpers as well if you plan on using them.
This calculator removes all guesswork, speculation and "well, it worked for us....". It will give you the precise performance data along with helpful graphs.