Thanks for all the input everyone! I decided to stop by a bike store with a couple of the options you guys suggested to give them a test drive. I got to try the Speed Uno and I was a big fan of it. I also tried the Mariner to compare it to a higher end model (in terms of cost and performance I definitely prefer the Speed Uno).
Dynocoaster - Do you know how much the R20 weighs?
Diode - That was definitely a point of thought/concern. Luckily I have an office on campus, so I should be good most of the time, and it seems like it is light/small enough that I can bring it to the classroom when I'm on the other side of Cambridge.