Well, my keeper league (in our 5th year) moved from Yahoo Plus to ESPN this year, and have so far been happy.
That said, it all just depends on the tastes & experience of the league. For example, if you have a bunch of folks who have never used ESPN, you might not want to uproot the league. If you're doing keepers, use ESPN. Just want a casual redraft for some newbies, use Yahoo free version.
We loved Yahoo, but the cost of the Plus league to run our keepers cleanly was insane next to ESPNs free service. Now, if ESPN charges for that next year or Yahoo does keepers free, we'll hop right back to Yahoo.
ESPN does offer a much more customizable page IMO though. Editing our own message boards, images, polls, etc, all make the league more fun.
So, to me it's:
Note: I've only used CBS & Fox once in the past and hated it, so take that with a grain of salt.
"Ya'll are brutalizing me!" - Ronwell Dobbs