You should be doing BOTH.
THE RULES OF STATIC STRETCHING
When: Any time of day, except before a workout
Why: To improve general flexibility
Arm Swings To loosen and warm up your shoulders, swing your left arm in large circles like a windmill. Swing your arm forward five times and then backward five times. Repeat six to eight times and then do the same with your right arm.
Standing Quad Stretch To loosen your quadriceps–the muscles along the front of your thighs–stand with your feet slightly apart. Keeping your thighs still and your knees slightly apart but next to each other, raise your lower left leg behind you, lifting the heel of your foot toward your butt. Grab the ankle of your left foot with your left hand, and gently pull your heel closer to your butt. If you have trouble keeping your balance, you can hold onto the back of a chair with your free hand. Hold for two to three seconds and repeat six to eight times, deepening the stretch with each repetition. Then do the stretch sequence on your right leg.
Position your body as shown and push your hips forward while keeping your torso upright.
Place your left hand on your head as shown and position your right arm behind your back. Gently pull your head toward your shoulder.
Position your body as shown, then push your arms into the Swiss ball and your chest toward the floor.
Position your body in front of a wall as shown and lean forward to stretch.
Place your arm against a door frame or wall as shown, then move your shoulder forward.
Place your foot on a sturdy box or bench as shown, then lean forward from the hips until you feel a stretch.
THE RULES OF DYNAMIC STRETCHING
When: As a warmup before any type of workout or sport
Why: To improve performance and reduce injury risk
How: Perform five to eight body-weight exercises or calisthenics at a slow tempo and in a comfortable range of motion. Increase your range and speed with each repetition, until you’re performing the movement quickly from start to finish. Do one set of 10 repetitions of each exercise, one after the other.
What: Try this sample routine of movements that are probably already familiar to you: jumping jacks, arm circles, trunk rotations, front lunges, side lunges, high knees, and body-weight squats.
Excerpts from the following Men’s Health articles: