I want to design a DC/DC converter Power supply, and I am choosing between MOSFET and BJT on which to use for Switching circuit. I have seen DC/DC converter switching IC that uses MOSFET/BJT for its switching part, which do you should I use? the one with MOSFET switch or BJT switch?
BJTs are now pretty obsolete for switching power supplies.
Some of the BJT designs you might still find in books are probably now forty years old.
The only reason you might want to use BJTs for a new design might be for a very low voltage low power application operating at perhaps only 1.5 volts (or less) which is below a workable gate threshold for efficient switching of most mosfets.
You you want some sensible power output, you should use a mosfet. They are not really expensive any more and they are well suited for switching applications. You can use a BJT if you wish a linear regulation (e.g., a series pass regulation). There are plenty of introductory books on this topic and most are excellent for the beginners.