Let’s determine the answer based on objectives:
You want to keep going
Cycling: Since cycling put lesser strain on your joints, it is considerably safe to cycle for a long period of time especially those with knee or back issues. Moreover, your stamina training is sustained.
Running: The duration of running doesn’t last long as it eventually leads to muscle tearing and soreness, putting a limit on how long you can go on your feet.
You want to build muscle mass
Cycling: Your quads, glutes and hamstrings are the foundation of your cycling power. As such, your leg muscles will increase as you cycle more. You should also lift some weights to keep your upper body proportionate.
Running: Even if you run uphill, it’s not possible to bulk-up by running. Your body becomes strained when you run. You’ll lean out your muscles as you burn fat, so if you’re trying to gain muscles, running isn’t the answer.
You want to trim your belly
Cycling: You can burn up to 100 calories just by cycling for 10 minutes. However, you’re not going to burn as much as you think, as cycling is a sit-down exercise. Therefore, minimal effort is exerted.
Running: You can burn up to 200 calories just by running for about 14 minutes. You can burn even more and much faster if you run uphill.
You don’t want to suffer an injury during training
Cycling: You can cycle for about 900,000 miles before your legs give way. Your legs can only take so much stress, but it will take a while before you suffer a cycling injury.
Running: Even running on flat terrains will send shocks to your spine, knee and other body parts. Aside from impacting your body each time your feet bounce up and down on concrete, your body temporary shrinks a little after a marathon race. You may even sprain your ankles when you step on an uneven surface.