For the lowest level courses, get the model you will eventually need in a later course (e.g., a student in MATH 1100 who will eventually take MATH 1160 should get a TI-83/84, but a student in MATH ...
iOS/Android/Desktop: Default calculator apps suck. They work like a traditional handheld calculator, which only displays one value at a time and can only do basic math. If you want to do anything more ...