A week ago, as I signed into Yahoo! Mail, I got an offer to use the Beta version of Yahoo! Mail. I also use Windows Live Mail Beta. What I like in Yahoo! though is that whevever you choose to open any email, it opens in a separate tab rather than opening in the same window like Windows Live does. But when it comes to loading and performance, Windows Live Mail rules the roost. Logging into Yahoo! is still slow and it sometimes even fails to login. It does not work on IE 7 or Opera (Windows Live atleast displays the basic version of the website). But it does give an option to use the old Yahoo! interface for the session.
We still need to wait for the final releases of both the services before saying who’s the best.