EMT will teach you some things you wont learn in Nursing school, which means it wont help much during nursing school, but might be slightly beneficial afterwards, especially if you want to be an ER Nurse.
Keep in mind that once you get your EMT license, you have to be affiliated with an EMS agency to keep it valid, which means either working as an EMT or at least serving with a volunteer agency. If you arent affiliated at the time your license needs to be renewed, you lose your EMT license, and it can be pretty difficult to get it back, and would probably require you to retake the entire course and pass the testing once again. National certification needs to be renewed every 2 years and state 3 years(for my state anyway). So if you're in a BSN program, are going to have the time to work as an EMT and keep up on your continuing education credits?
Of course if you just want to take the class to learn something new and have no intention of later being a flight medic or whatever, this is irrelevant.
Also keep in mind EMT school isnt a blow off class. Its pretty time consuming and expensive once you add up tuition, the cost of books, uniforms, lab fees, etc, and a surprising number of people fail it. So if you plan on taking other classes that semester, Id go with CNA school, which is a lot easier.