I don't think it takes a genius to figure out which is easier to perform (smiling of course!). So why use more energy to move those frowning muscles to, well, frown?
I know, I know, it's difficult to hold that smile for a minute for picture day. It's quite painful. I know. But if you smile, you actually get happier. Try smiling when you're upset. It helps a lot. I honestly don't know why. It just... Works.
Also, there's a quote that I think I remember it's something along the lines of "every minute that you frown is a minute of happiness you will never get back." So don't frown. Frowning is bad. Yeah. Say no to frowning.