From a workout standpoint, both are great options. Indoor cycling is best for people that want motivation in a group setting. It requires less skill, as you don't have to navigate through traffic, an open road, or a mountain. It can be more convenient for some people as well. But, it can get boring fast. Outdoor cycling can be more stimulating. But, it requires more skill than indoor cycling. It also needs special gear: the bike itself, clothes, shoes, helmet, etc. which can be expensive to some people.