Shoot Vertically for More Impact
Much of pro sports photography is shot vertically because it's easier to fill the frame with your subject (plus, it's ideal for magazine covers, ads, etc.). Just turn the camera sideways, get in tight (with your long lens) and make the magic happen (so to speak). This particularly holds true if you're shooting a single athlete, rather than two or more, where a horizontal shot might work best, but when it comes to shooting a single subject, your best bet is to go vertical. That being said, if you want to really cover your bases (like the way I worked that sports metaphor in there), shoot both as much as possible. As my tech editor Bill Fortney says, "The editor will always ask for the orientation you didn't shoot."