Exam boards require a scientific calculator that is not a graphical model with symbolic algebra. Beyond that, the practical question is which functions your specification expects you to use.
Functions worth having
- Natural display of fractions, surds and powers, so answers can be read in exact form.
- Statistical mode for mean, standard deviation and, at A-Level, binomial and normal distribution functions.
- Table mode for evaluating a function at several values quickly.
- Equation solving for quadratics and simultaneous equations, useful for checking rather than replacing method.
Practise on the calculator you will sit the exam with
Every model has slightly different key sequences for distributions, memory recall and mode switching. Borrowing a different calculator on exam day costs time that has nothing to do with mathematics.
Where the calculator should not be used
On 'show that', 'prove' and exact-answer questions, the working earns the marks. A correct decimal where an exact surd was asked for will not score, and a result with no method behind it can score less than a partially correct attempt.