There are pros and cons for both types. You should fly whatever your school is teaching you.
IMO, square ccts are better when beginning to give the student more time to sort themselves out on each leg and to set spacing when turning downwind. They also allow for better visibility before turning each leg and better judgement of base aspect.
Rounded ccts, while taught initially with some schools, are more efficient and will come with practise. They are flown by all the airlines and (as already stated) the ADF.
There is really no 100% correct answer to this question.