The Shimano Hyperglide C cassette on my Trek 470 road bike is a bit worn. I would like to replace it, but I am not sure what tooth combination to choose. The original cassette is a 7 speed 11T - 24T. Between Nashbar, AEbike and other vendors, I have seen 12-32, 11-28, 13-26, 12-28, etc.

I do most of my riding on a paved 25 mile rail trail that is fairly flat with some small inclines. What cassette combination would you recommend for me?

Thank you.