Both have their uses. In my opinion, an established alternate picking technique gives a more even tempo, where economy picking I find myself slightly faster changing strings if I'm not paying attention (!)

Economy picking can be faster depending on the lick so as schmange says, it's all economy of movement. If you get it in to your head that you want to economy pick all the time, you have to work your licks out so they flow correctly - it needs more planning.

As the guys say, learn both to the extent that when you're flying you're not totally sure which you're doing but you know it's working :D