Depends on your priorities. YMMV. Cairns has better hotels (though still average by southern standards) - Hilton, Shangri-la, Pullman International, Pullman Reef Casino, Harbour Lights. It has better restaurants - Tamarind, C'est Bon, Prawn Star (and easier access to Palm Cove which has several very good restaurants). PD does have Salsa, one of my all time favourites.

Cairns also has a huge supply of holiday apartments if you prefer those to hotels. And Cairns has far more varied access to the reef compared with PD. PD is nearer the rainforests if that is your interest. Either way, have a great time. If you want a laid back town where everyone is visiting PD is the one and a pleasant one at that.