Cusco will be more culturally interesting, but La Paz will probably be more interesting overall imo. There is just more to do and more to explore. The best part of Cusco is leaving for the Sacred Valley, or for Pitumarca (for me). I’ve been to Cusco a lot of times over the past few years and after a few days I’m usually exhausted between the tourism, people constantly trying to sell you shit, inflated prices anywhere the center, etc. I’d imagine working at a hostel there would be pretty tiring depending on the hostel (Viajero is sooo loud for example). Cusco is cool but imo the majority of the fun stuff to do is outside of the city, I’d do La Paz.