About the same. The 7700k would hold an edge.
6700k and 7700k both fit the same mobo. Both are lga1151. The only caveat is that on older skylake boards that have not seen a bios update since kabylake was released, the kabylake cpus won't be supported. A simple bios update, (prior to adding the kabylake cpu) fixes that. A 7700k will fit, no worries