HD aftermarket springs or if you can find a place to do it, have an extra leaf added to your factory set. Also get some decent big bore shocks - factory shocks are usually rubbish and don't last long.
There are some horror stories regarding the use of airbag suspension but there are certainly plenty of people that use them and are very happy with the job they do. Horror stories usually involve too much weight behind the rear axle and towing, too much pressure in the airbags and then some sort hard impact such as hitting a washout on the beach at speed. I have airbags in my navara and really like the versatility of them. I have a canopy on the bag with fridge, drawers, battery etc and regularly tow a heavy camper or 2Tonne boat with no dramas.
Definitely DO NOT just install airbags with factory springs and expect them to do all the work for you. Also, some dual cabs have more strength/support in their chassis rails than others and some have longer trays/towbars than others which can increase the liklihood of problems occuring.