First decide what is your preference consodering many factors such as Money,Passion, job security, Foreign studies, classmates, time dedication, Interests etc
Then by Analysing all the alternatives choose the best course and branch.
Money – CS
BIT has very good placements for EC CS and IS. If you are above average you are placed for sure with a good package. The job security is low as you never know what happens with new technology and new roles. Good for further studies as you can go to USA and there is plenty of scope there. Time dedication is high and you have to byheart a lot of uninteresting useless subjects. Being passionate for CS will not fetch you marks and you will have to go the conventional way so balance everything in between.
Mechanical is a more interesting branch with less to study. The core placements are for the top guys and they have to roam a lot for almost half the money. Job security is high and as you learn ,after 15 years you will be better placed compared to others. You can write GATE be placed in a PSU and many other non monetory advantages. No Girls in class but doesn’t matter that much.