For example, you ride for a constant 30 minutes on the stationary bike at a set tension and constant intensity
You ride on the bicycle for 30 minutes, but not at a set tension and not a constant intensity, i.e., you coast for 5 minutes and ride uphill for 10 minutes putting more intensity into riding uphill.
Which is better?
It all depends on what your goals are. If you are training for a particular event/rac and you want to improve your cardiovascular endurance, you would want to train more so with the bicycle. If you have to ride the stationary bike, whether pending or you just want to switch up the routine, you will want to constantly change your intensity by doing short bursts of going all out and then lowering the tension and staying at a set intensity and so on in this manner. This training is what you should be doing if your goals are to gain cardiovascular endurance and improve your max VO2. This is known as interval training and is great. Remember, go fast and relax! Go fast and relax!
When you just go at a set pace, your muscles eventually get used to it and therefore they do not grow. You need to change your intensity all the time