Personally, I suggest riding a bike, mainly considering the degree of injury to the knee joint. It is better to ride than to run.

As for the effect of slimming, it depends on the amount of exercise you do per unit time. There's a BBC survey. Try to ride a bike for 3 minutes every week, which can meet the amount of exercise for a week. So strength is the most important thing.

Riding a bike is more fun, but pay attention to safety.