Walmart and other big-box store bicycles are infamous in the cycling community for their poor quality and assembly, often to the point of being completely unsafe to ride at any speed (Nader reference intended).
They often use the cheapest high-tensile steel they can find, off-brand rims, mix-and-match groupsets, low-quality brakes, and so forth. The bike you linked exemplifies that - mixed Shimano/SRAM components and "alloy wheels" are in its description. Box-store bikes are also never put together correctly - flipped forks, pinched inner tubes, and sheared screws/bolts abound when they come in for repairs.
Comparing these two bicycles directly, the big things that stand out as pros are the Sladda's aluminum frame and disc brakes. An internal gear hub is also a nice low-maintenance touch. It remains to be be seen how well the Sladda holds up, but $400-500 isn't too unreasonable for a aluminum city-cruiser that's been ruggedized.