Hey cherry
Both of these branches have different and unique essence. The answer totally depends upon your interest, capability, talent, grasping power and inclination.
If you are kind of guy who is good with not just computers but with its soul, i.e., hardware, software and most importantly programming languages (C, C+, VB, .NET, Java, etc.), and you don't get frustrated with complications of these, instead enjoy dealing with them, then, Computer Science is the branch for you.
If you are the kind of guy who gets fascinated by the mechanics of everything that you see in your day to day life, say for example, the way your bike behaves in odd or good manners, the way steering acts, gearing mechanisms, machinery, etc. then mechanical engineering is the branch for you.
One of the most important and interesting fact about Mechanical Engineering is that it consists fundas of each and every branch, be it chemical, electrical, electronics, computer science, physics, etc. If you are good in mathematics then you won't have much problem in solving numericals in mechanical.
Mostly computer science engineers get the white collared jobs whereas mechanical engineers might get white collared job if he/she is in design field or might get field work.
So before choosing the branch you need to understand the kind jobs you might get after graduation and whether you are actually enjoying the course of learning without any kind of frustration or not.
Although in India mostly people choose either of these branches only but before making any decision check out the various branches, the kind of job opportunities they offer in India or in any other country (if you have no problem going abroad) and most importantly which branch is appealing to you the most. After all you need to be in the same branch (if you don't plan to change your stream in second year).
P.S.: don't choose on the sole criteria of money, choose based upon your interest. An average paying job might give immense pleasure to an individual if the stream and job are of his/her interest but the highest paying job might frustrate an individual so much if the job and stream are not of his interest.
Hope it helps
Thank-you
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