An academic year offers the opportunity for the fullest possible adaptation and integration. You will invest so much time and effort the first semester into learning about your surroundings and the culture, why leave before you can reap the benefits in the second semester?
While many programs allow you to apply for one semester (rather than an academic year) - we suggest applying for the academic year if you're on the fence. (In many cases it's more complicated, or not even possible, to extend from a semester than a year, than to shorten your program from an academic year to a semester after you get overseas.)