The main difference between Maxwell and GCN is that GCN is a massively parallel architecture with higher compute power and the ability to perform more tasks asynchronously from the graphics queue. This difference is not going to be noticed until DX12 and Vulkan take foothold. Then people are going to realize just how gimped of compute power Maxwell 2 is.
Hell, even AMD's years old cards support feature level 12_0 in DX12 and nothing before Maxwell 2 supports it. Nvidia's claims that Maxwell 2 is the only one that supports full DX12 is also FUD since they don't support resource binding tier 3, it does however support feature level 12_1 which so far none of AMD's cards have been confirmed to support.
This is good article explaining an architectural lead AMD has over Nvidia going into the DX12 and Vulkan era:
http://www.anandtech.com/show/9124/amd-dives-deep-on-asynchronous-shading