Hello aspirant,

Generally candidates opt for PCM with biology only if they have a strong grip over mathematics or they want to attempt the NEET exam as well as JEE exam. If you wish to do something in the medical field such as MBBS, BDS, BAMS or BHMS, you can give the NEET exam.
If you wish to do pursue something in the engineering field, you can attempt the JEE exam. However, if you are unsure about what course you would like to take up further in future, you can attempt both the exams and see for yourself what you wish to do in the future and where you have scored better marks. Though marks do not matter if really want to do something, they can act as an indicator sometimes as to where you can have a better scope of building a better future.

Hope it helps! All the best!