My own personal experiences with both:
Ease of use? Probably Flux.
Performance? Knet.
Not sure if this is unique to me, but my custom models with large 3D data, an epoch on a single core of my K80 takes 35 minutes in Flux. The same model in Knet with the recent CUDA changes takes 10 minutes. YMMV.
Flux comes “ready to go,” whereas Knet performs the underlying NN computations but you have to define your own layers (not too big of a deal to be honest). The maintainer of Knet is very active and is constantly updating the package. In fact I think he’s working on implementing a layers interface, so that should be available soon.