treadmill will give you more of an all round work out but when it comes to running on a treadmill there is less space to do other running workouts so you are forced to jog or sprint if you ran in real life like outside you could do high knees karaokes butt kickers lunges or leap jumps.
with an exercise bike you will get an easier cardio workout that will clean your body and keep you healthy, you can also stand up on exercise bike and transition speed a lot better which will work your legs differently to get a better workout.