[Update: September, 2017 comparison w/11 cameras]
To view in 6K with dynamic streaming / adaptive viewport, click through to the post in Facebook, "Save Post" or "Save Video", and view the video in the Oculus Videos app on your Gear VR.
Cameras were tripod-mounted and shot using default modes and exposures. The resulting videos were placed in a 29.97 fps timeline in Adobe Premiere Pro and upscaled to 6K to match the highest-resolution source camera (Z CAM S1). To match orientations, all videos were rotated around the y-axis using the Mettle SkyBox Rotate Sphere plugin. Export was done in 6K to DNxHR, followed by a command-line export to 6K H.264 using ffmpeg (6K video exceeds the maximum spec of H.264's High Profile, Level 5.2, but ffmpeg allows it).
You can download 4K and 6K versions of this video here: http://ech.cc/360camdownload.
The 4K version is suitable for sideloading into a Gear VR. The 6K version is difficult to view locally, but I've been doing it by viewing a 6K Quicktime / Cineform version in GoPro VR Player 2.0.
Note that creating 360 content can be a complex process, and video quality is only one metric you should use in determining which camera is best for you. Photo quality, workflow, target distribution platforms, and ease of sharing are some other metrics that might be of interest.
Still image comparison
Here is a comparison of still images from 8 current 360 cameras including the Giroptic iO (pre-production), Insta360 Nano, LG 360 Cam, Nikon KeyMission 360, Panono, Ricoh Theta S, Samsung Gear 360 and Z CAM S1 (screengrab from 6K video).
Note that creating 360 content can be a complex process, and still image quality is only one metric you should use in determining which camera is best for you. Video quality, workflow, target distribution platform, and ease of sharing are some other metrics that might be of interest. Also, some 360 cameras are designed to be video first, with photo quality as a secondary priority.
Cameras were tripod-mounted and shot using default modes and exposures (except for Ricoh Theta S, which was shot in 3 different modes all with default exposure). Still images were opened in Photoshop, offset losslessly to re-center to match, and saved with a text label applied (labels were sized to match the original photo resolution and warped into place using Flexify 2 FB3x2 cube map -> equirectangular). Full-res images along with original images from camera are available for download here: http://ech.cc/360camdownload
To view these in full quality, download the original files and view them using a local 360 photo viewer like GoPro VR Player.
If you'd like to discuss the results more publicly, the comparison has been shared to the Facebook 360 Community group.
Note that mounting cameras like the Giroptic iO and Insta360 Nano/Air requires a smartphone tripod adapter, and in the case of Insta360 cameras, can be tricky because the phone must be mounted backward.