Yahoo, no question. ESPN doesn’t really keep their system up to date and their rankings are absolutely trash. They will take forever to update positions or add new players from the G-league/overseas. I also think the UI is much better on Yahoo. As for format, H2H categories is the way to go.
https://www.reddit.com/r/fantasybball/comments/q2cz4q/yahoo_fantasy_vs_espn_fantasy/

