eLitmus is a test geared towards a wider range of Engineering, business or tech grads/post grads. AMCAT however is more a measure of command on computer skills and knowledge. So for a mechanical engineer, eLitmus will be a more suitable test when it comes to hiring and job opportunities. If your interest lies in Computer science or computer applications and you are proficient in it, you can opt to write AMCAT as well. Hope this helps.
Good luck.