Strength training is better for boosting your metabolism.
Muscle IS your metabolism—the more muscle mass you have, the more calories you burn at rest, period. Plus, strength training creates a level of muscle damage that increases metabolism post-exercise during the recovery and repair process. Yes, intensive and prolonged periods of running and cycling can also create muscle damage and generate a post-workout afterburn, but not at the level of regular resistance work and certainly not for your whole body.
In addition, a recent study from North Dakota State University showed that you can burn 346 calories in just 13 minutes of a simple 6-exercise resistance circuit. Compare that to the 146 calories you can burn running at a 10-minute mile pace for the same amount of time. And it would be even less for cycling since less of your body is involved. I modeled the workouts in my Men's Health MetaShred Extreme body transformation program after the fat-frying routine used in the North Dakota State study. Check it out; the results speak for themselves.
Higher-intensity total-body resistance training is better for burning calories because it increases both anaerobic and aerobic energy expenditures. This means that you burn calories both during and after your workout. When you do long bouts of steady state cardio, the calorie-burn stops the moment you stop. You can remedy some of this by incorporating HIIT (high-intensity interval training) into your running or cycling workouts.
And if your goal is just to get your heart rate up as high as possible, try doing things like burpees, kettlebell swings, thrusters, and battle ropes. Go all out for 60 seconds and your heart rate will get just as high as with max-effort running or cycling. In fact, a recent study showed that 12-minutes of kettlebell swings had the same cardio and metabolic impact as running for the same period of time, but with the added benefit of being lower-impact and strengthening the often neglected muscles of your backside.
Should I stop now, or do you want me to keep going? Fine…