Under the 4WD drivetrain family, a double cab pickup is similar to a full wagon in that they both have real chassis, low range and diff-lock drivetrains, ride on large wheels and have a big clearance from the ground which is useful when driving over ungraded roads. Their structural differences are marginal but have huge implications on usefulness.
Travel ready, security and dust
Depending on the kind and dimensions of your luggage, you could trow anything in the trunk of a double-cab pickup. In a wagon, due to sealed doors, you would need to choose your luggage to ensure that the cabin remains drive-able. Odourless luggage is best of a wagon as compared to a double-cab pickup. On the contrary, the sealed nature of a wagon body provides a layer of security to your luggage while keeping dust away. Wagons seal a lot better and eliminate dust from getting into your luggage. This would not be a problem is you are ferrying farm produce, or equipment on the back of a pickup.
All leaning seats is a huge comfort booster in wagons as compared to uprights back seats in a double cabin pickup. Furthermore, wagons use better suspension in the rear axle which consists of coil springs and big girth-ed shocks for added travel comfort.
Double cabin pickups have leaf spring in the rear, allowing them to carry more weight compared to the wagon coil spring suspension.
Off road drive-ability
The rear wheels of wagons articulate better due to coil spring suspension. This enables wagons to have a longer suspension reach hence better grip as compared to leaf spring-ed double cabin pickups.
Double cabin pickups provide a modify-able bed that could be turned into anything.
The strong points
Wagons are comfortable, secure and easy to get off-road ready. On the other hand, double cabin pickups are endlessly customisable, and can handle heavy loads.
It really boils down to personal preferences. The double cabin pickup has strong points that would help, say a farmer tackle small tasks here and there, however, it would be a nightmare for a city dweller ferrying supplies and getting stuck in traffic along Ngara area in Nairobi.