I just used 11-28 as an example. I have no idea what I actually have (other than a 9 speed). I do know that I never use the small front gear. I like your explanation of not having a dramatic difference in your cadence with 12-21. I'd definitely like that. When I do replace the cassette I'll probably go more towards that/those sizes.

P.S. now that I know what those numbers mean, I'm looking forward to finding out what I do have when I get home this evening.