Use which ever one you are already most comfortable with. It is not worth the effort to get used to a new one and the gain will be marginal.
While the HP and RPN might be used at some banks (I can say that I have seen it) if that is the case you can pick it up after the exam. No need to force yourself to learn something for an exam. If you think you need to use the HP to fit in then learn it afterwards where you can practice without pressure.