Nobody can answer your question.Each of them has its own features and some people like gmail,some prefer yahoo.
i vote for yahoo mail instead.Love gmail also but i had use yahoo mail for 6 years and this relationship is strong between us.
Definitely Gmail. I'm a fan of Gmail. As for Yahoo mail, I don't like all those annoying banners and picture ads.
Well if you are asking from space and usability point of view then obviously Gmail is better than yahoo mail, but yahoo is also thinking to defeat through providing more space than Gmail. More information can be found here http://yodel.yahoo.com/2007/03/27/yahoo-mail-goes-to-infinity-and-beyond
I use my gmail account the most, all my blogs posts are stored there too, anything that keeps growing is good.