If you have to choose, then you should exercise harder instead of longer. Studies have shown that shorter bouts of higher intensity exercise are more beneficial than a longer but moderately-paced workout. The results apply to both disease prevention and weight loss benefits.
Any form of activity, whether long and easy or short and hard, is better than no activity at all, of course. Studies have also shown that walking for at least 30 minutes a day, every day, is also good for your health. Whether you’re aiming for 2,000 or 10,000 steps a day, or anything in between, the important thing is that you’re physically active.