which of the two is better to use, if both are used for same amount of time?
like to burn fat, to tone up muscle, etc?
Personally I would never reccomend the tredmill - I won't go into details but it screws up gait and can cause lower back problems.
The bike can be ok... just be aware that if you already have tight hip flexors that you will want to stretch them out before and after the session.
For fat loss I have said a thousand times that the "fat burning zone" is old news - intervial training or HIIT is best for fat loss when looking at "cardio".
What is the best way for burning thigh can calf fat w/o developing muscles eh?