Course comparison

Put it this way;
A chemical engineer is in the laboratory developing a new drug for BioX Pharmaceuticals. His team has been at work some time now trying to synthesize the new drug, and finally one day has a breakthrough. After patenting, having the medical scientists perform trials, and getting the FDA to approve the new drug BioX decides to put it into production. The engineer, working alongside other engineers, may develop an automatic system to manufacture the drug. Someone has to monitor the manufacturing system; programme the machines, make sure they are working well, and replace the chemicals needed to manufacture the drug. That's where the Technologist comes in and that's what they do, depending on the level of their education. Some techs may just programme or maintain the machines and chemicals. Others are in management positions, sales, and applied engineering. Still, others may work as lab assistants to the engineers. Working the instruments, setting up experiments, and collecting data. This is what most chem techs probably do.

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.