I borrowed my brother's HP-12 calculator for the exam, and I really like it. But I had already studied some RPN methodology back in high school so I was somewhat familiar with it. As someone said above, RPN is very efficient for calculating multiple-step or complicated problems. But depending on how quantitative your brain is, learning RPN can be of varying degrees of difficulty.
If you think your brain works in a fairly numerical way, I'd suggest the HP-12 (or HP-12C), as RPN just kind of "makes sense" after you learn it. But it might feel kind of strange at first. Ie, you don't really see the benefits of RPN until you start to delve into calculating complex expressions.
If you want something very user-friendly that doesn't have a steeper learning curve, than I'd suggest going with the BA-II. I haven't used it, but from what I understand, BA-II has a nice screen that lets you see what variables you've defined and what you're typing as you go along, whereas the HP-12 only shows you one 10-digit number at a time.