Window wall is a cheaper system that sits on the slab edge, but provides far less insulation at the slab edge (usually about 1" max) and requires a membrane tie-in at every single floor. The installer has to be on their game at every tie-in or it will leak eventually. Curtain wall is supported off the edge of the slab, uses stronger, thicker extrusions and has thicker insulation at the slab edge (the depth of the back body of the mullion is usually mineral wool insulation) plus there is about a 1" gap between the mullion and slab edge that gets filled with more mineral wool insulation). In addition, the curtain wall system is completely face sealed, so there are no membranes to tie in at every floor. Curtain wall is a far superior system than window wall, they look the same, but looks can be deceiving. The problem is curtain wall goes for about $150/square foot whereas window wall is closer to $80, so it's all about cost. Curtain wall will outlive a window wall by a lot, but most developers don't care what happens to their projects in the future, they get their money and get out leaving repair costs to the owners.
