Thank you very much for all your great advice. I seem to spend most of my time in the 12t - 18t range. So, I will probably opt for whatever cassette is still available in the 12t - 28t range. Looks like they are all about $20 - $30.

Is there any difference in quality between Shimano, Sram, Nashbar, etc.? What brands do you recommend? Thank you.