Mechatronics Engineering

It depends on many things. (I'm answering this based on the assumption that you are asking it on the level of BE/B.Tech)
Go for mechatronics, if-
You want to study and explore multi-domain engineering fields such as mechanical, electrical, electronics, controls, computer Science etc. You have got it as B.Tech in a very good college. If the college is having good placements. You don't have plans to study other specializations in your Masters apart from mechatronics (as there will be very few options left if your B.Tech in Mechatronics).
Go for mechanical, if-
Your not sure where you'll end up with which mechanical field. You want to study many things related to mechanical engineering. You like to and have interest in machines and automobiles. You want to do your Masters in future. Having B.Tech in mechanical opens many options for specialisation (including mechatronics). You want to get into placements of core mechanical companies or wants to gives exams such as GATE/ESE/ISRO/DRDO/IOCL, state technical exams etc. This is bit hard if you opt for B.Tech Mechatronics, as most of the time criteria is of mechanical only. Girls don't matter to you much but your studies and career.
So, the choice is yours. Do research more and take decision. Good Luck.

Hi
Both Mechatronics and Biotechnology are niche branches. But unfortunately, in India, we have less scope and opportunities. Unless you do these courses in very reputed institutes, you will not be placed in reputed companies. For mechatronics, opportunities are open in Army, Navy, Electronics, Automobile, Gas manufacturing and ISRO. This is a very niche branch and has a lot of career and education opportunities abroad. Biotechnology is more research oriented here also you need to go for post graduation in well reputed institutes to enhance and do the real job in the biotechnology field. Otherwise, we end up in doing a clerk jobs in service organizations. Mechatronics is a better option than biotechnology.

The field of mechatronics is very promising, when it comes to job opportunities. Mechatronics is a combination of various fields of engineering. Due to this very fact, the field of mechatronics has wide areas of application. Thanks to the availability of many areas of applications, career prospects associated with this field is also great. Some areas where the knowledge of mechatronics is applied are-
Robotics
Telecom sector
Mechanical systems
Electrical systems
Biomedical systems
Automation
Nanotechnology
Metallurgy
Biomechatronics
Due to availability of large number of areas of applications, mechatronics graduates have large number of job types to choose from! It is safe to say that mechatronics graduates are very versatile. They are capable of taking up jobs from more than one engineering branch! With further specialisation, through PG courses, they may choose a particular discipline and specialize in it. Some such areas of specialisation are- Robotics, Biomechatronics, Nanotechnology and Programming. Regards.