In fact they are both commonly used ! 'Bravo' is widely used as an appreciation at the end of a performance or show as Caroline mentioned. You seldom find people saying ' tres bien' at the end of a musical or opera for example . "Tres bien" is more commonly used to praise an act ,a good quality but lesser for object appreciation , like you won't see a nice painting and comment ' tres bien' , but rather use it to praise a nice meal for example . If someone performed an act in front of you , like a dance , a movement or showed you a finished piece of art ,,,, if you said " tres bien' you refer to the quality of the their product or their performance in that case. If you used 'bravo' it is sort of an appreciation and encouragement at the same time .

bravo is like clapping hands after a show. Très bien means "very good". They are similar in meaning both are popular to show appreciation.

Les 2 !