I believe you'll be completely happy with ESPN. I've tried CBS, ESPN, and Yahoo for several years and prefer ESPN. Here is a link to their 2009 auction rules. The rules for 2010 should be similar:http://games.espn.go.com/flb/content?page=flbrulesauctionoverview2009
The customization does allow for very deep benches (2C, MI, CI, etc) and ESPN supports up to 20 teams. I've commished five leagues there over the past two years and four of them were 20-team leagues. I've never had to pay for any customizations.
If ESPN doesn't dramatically change things for 2010, I believe you'll be quite satisfied with a move there.