In terms of picture quality and picture frame look, the frame is top.
The software for displaying photos in art mode is very limited. The same picture is always displayed until you change it manually. Up to 1 GB of memory can be stored in the frame, but the frame does not make a slideshow in Art Mode. For me, for example, it would be nice if the frame would randomly select and display an image from the stored collection every time it is reactivated by the motion sensor. Maybe such a feature will come with a software update.
Regardless of the 4K display, so it doesn't necessarily have to be a frame, I think an Apple TV is a nice solution for displaying images. Either in a slideshow mode of Apple's Photo App or also, for example, of Lightroom, or then simply in screensaver mode, in which random images from a previously defined album of photos are shown. Here, however, there is a small Apple hook (as of today 05.12.2017): Only one shared album can be selected as a screensaver and according to Apple the maximum resolution for photos in a shared album is not 4K, but something between 4K and Full HD. For display on a 4K display, the images are scaled up again. I think you can see the difference between a 4K rendered image from Lightroom and the same upscaled image from Photos quite clearly. Of course, it varies somewhat from image to image.