Biotechnology is a vast field where as microbiology is limited to microorganisms, but no doubt if you want to get into research or want to became scientist then microbiology is a better option.

Having said that having a Biotechnology degree opens up many more doors, in various fields like industrial processes, management jobs while microbiology jobs are more research oriented. You also have an option of doing an MBA in Biotechnology, which a new and emerging field of work.