It’s a very difficult question - what is better, because no matter what calculator do you take - you will be able to pass all three CFA exams. That’s the fact.
Before you buy the calculator, you should answer yourself for the very simple and basic question.
What do I preffer?
Do I preffer a traditional calculator and I like counting the way they do? Or, do I want to calculate the way I count in my mind?
If your answer is YES for the first question take BA, if for second - buy HP.
From my point of view.
I have HP and I think this is the best calculator I’ve ever had. It uses Reverse Polish Notation what is similar to the way you calculate in your mind. RPN is difficult on the beginning, but when you make some practice - you can do all calculations much faster than on traditional calculator.
If you take HP 12c platinum - you can choose; RPN or traditional algerabic system.