I tend to use the latter because the former only pops out a small version in Gmail, the latter is a shortcut to open the full site in a new tab.
Related - In Outlook.com's interface a similar function exists for the calendar there - BUT it opens the shortcut in the same tab, taking you out of your inbox, instead of opening a new tab. It's frustrating to learn the 2 behaviours for the same type of shortcut. Who wants to close their inbox when opening their calendar?