Depending on your perception of an email package solution, you may want to decide to go with either or. Before you select either or keep into account that both, Yahoo and Gmail are known for having privacy issues which have been exposed all over the web. With that said, here's my recomendation.
If you enjoy a user friendly interface and like to take full control of your emails by placing them in folders with tags to help you recognize emails you've gotten then by all means choose Gmail.
On the other hand, if you like a more conservative, add oriented interface with a bit more of serious tone to it then select Yahoo.
There are important benefits by selecting Gmail among them space. Gmail keeps adding space to its free email users on a constant basis where as Yahoo gives you a set amount of email space which you can use at will.
Security wise you're better off selecting other options such as hmail, gmx or even inbox.com. This however will come at a price if you want to go pro.
