Before you make a purchase, think about your budget and what you really want.  If you're wearing the undershirt to the gym or a labor-intensive job, then an athletic style may be the best way to go.  if you just want something functional and economical and you don't care how it looks, pick up a pack of good ol' generic white cotton under tees.  It's important to choose something comfortable, too.  If an undershirt is uncomfortable, it's the wrong fit, the wrong material, or both).