I have done a bit of research on this.
As the same dilema is going on with me.
I have talked to many seniors in BITS
They have said its possible to study in BITS but the environment, frens etc make it a bit tough.
I would advice you to join IIT and study there and have a branch change coz the IIT tag does matters but yes just dont go for this Branch change.
Considering Biotech i had talked to professors there and they say it has got more application of chemistry and less from Bio side.
So it is a safe branch to go with.
But if you dont want it the best option would be BITS EEE but do not compromise IIT for BITS mech.