You should do M.Sc in Astronomy as it's a specialization in a particular field . You have a doubt what you should do and right now your mind is in contradiction but your thoughts needs to be clear that which stream you want to pursue .The Astronomy would be better for you as it would allow you to only research what you like .This will help you to learn more about the subject which you like so much . This would polish you to become a good astronomer .