Hi Amit.
Nice that you're going to buy a BMW, and we will be glad to help you with it!
My first question: what year BMW are you looking for? and do you know the e-number? (
you can have a look here if you dont know)
I can tell you that the 6cylinders are certainly more thirsty, but you'll also notice that the 6cyl will be more powerful.
If you tell me which 318/320 you're looking into, I can be of more assistence!
Ilir