NERSC's Cori machine is the biggest machine of Knight's Landing (10,000 nodes), 10th on TOP500. Don't know of any planned Knight's Mill machines.

Generally, Xeon Phis get fewer FLOPs than GPUs but are better at certain tasks, e.g. graph algorithms with lots of divergent control flow. Also it's possible to run pure MPI programs on them and get decent performance, and there's a lot of pure-MPI HPC software out there.