Ask your doctor to show you exactly where the port would be located in each case. The positioning it to provide proximity to where the catheter at the other end terminates in a major artery. Most people have the port placement in their upper chest just below the color bonem which is where mine is located. Mine has never been a problem, but some people say it is right where the shoulder strap of a car seat belt goes and that is sometimes uncomfortable. On the other hand having it in your arm would depend on where in the arm. If on top, it might be more noticeable if that is a concern for you. If on the side or bottom, that could be inconvenient when getting dressed or certain positions and movements if it can rub in the armpit area or against your body.
One other comment. The type of access system typically put in the arm is called a PICC line. While it is used for the same purpose as a port, the actual mechanism is quite different and it terminates externally rather than under the skin.