For me, I would prefer using Gmail and here is why:
First off, Simplicity. If you observe and compare the two mailing sites, Gmail undeniably looks minimalist and user-friendly. There is no complicated tabs that will pop up whenever you click a mail in your inbox. For me Gmail has the upper hand in this one.
Secondly, I like the convenience Google has to offer. Since it is linked to Google, Gmail gives you a "One click away" feature when you are, for example, downloading a game or verifying a purchase. Majority of sites are universally linked to Google to give you a convenient user experience.
Lastly, I don't know if you know this but Yahoo is currently experiencing major breakdowns. I disabled my email account because of the frequent spam messages and gags that "floods" my inbox. As a user, I find it irritating and inconvenient.