Media Viewer and Commons slideshow have similarities (both display images, information about images and allow to navigate through a sequence of images). However, they have different purposes and those get reflected in their layout choices:
Commons slideshow shows the collection of images and detailed information for the current file. It also provides controls to move through the collection and play/pause the slideshow. That is useful to view all kinds of details of a group of images, but the area to actually view the image becomes reduced as a consequence. As Gergo mentioned, we have no data from actual use on how much that impact users.
Media Viewer tries to maximise the space devoted to show the image and places it as the first most prominent element. It provides support even go completely full-screen and use the 100% of your screen (as the image aspect ratio allows) for viewing the image. Media viewer facilitates viewing images while providing basic actions that the casual user needs while viewing an image without the need for additional navigation, as exemplified below.
Moving through a collection of images is important for many different purposes (our data shows that about 35% of image views for Media Viewer are produced by using next/previous controls). In addition of media display purposes, Media Viewer provides shortcuts about the next logical steps to do once you viewed an image (e.g., download, share or reuse) and guidance (e.g., how to attribute). This has been optimised for repetition when going through a sequence of images by allowing to keep the download panel open. If you imagine going through the
picture of the year collection and saving all images you like, you can see that Media Viewer avoids some extra navigation steps between the collection and the detailed view (being it the File Page or the Slideshow).
Pau