amidst is an uncommon word. Among is pretty common. Their meaning is similar when they are interchangeable. Amidst means 'in the MIDdle of some other things'. It is only applied to the physical location of objects. Among means 'one of a group of things'. It is often used in logical statements, numbers or counting statements. Example where the use is the same 'I was standing amidst the students' / 'I was standing among the students'. Other than the speaker probably being more towards the centre of the group, there is not much difference. Example where they are not interchangeable. "who among you has completed the test?" is like saying 'out of all of you, which has completed the test'.

amidst and among mean the same thing. You can use them , it depends upon you. Most people use the word among. If you are speaking to friends I would use among. If you were speaking at a Business meeting , you could use the word amidst .