I've been ro both, my personal opinion, la jenny.
Nicer beach, more activities, nicer night life, and drom what i remember more 20-30 years old people.
Bur if u have a choice I'd recommend arna, beautiful beach, enough activities to keep u busy, all activities are free including the gym.
At night (00:00-02:00) there is a dance bar and rhe place is full of 20-30 yo, but u'll probably see them mostly in the bar at night.
My friends r also going there and unfortunately this year i might not b able to join them ????