When comparing photoqt vs Phototonic, the Slant community recommends photoqt for most people. In the question“What are the best FOSS image viewers for UNIX-like systems?” photoqt is ranked 12th while Phototonic is ranked 15th.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Highly customizable
Pro Support many image formats
Pro Simple
Pro Clean and functional UI
Beside being pretty clean and easy-to-use, the UI is also pretty fast.
Pro Can be easily keyboard driven
Lots of pre-defined keyboard shortcuts.
Pro Highly customizable
Lots of options to personalize your experience. You can set Photonic to fit images by width and height, or width or height; rotate according to Exif orientation; show/hide image name in viewer; adjust space between thumbnails; adjust delay between slides.
Cons
Con Windowed mode is a little bit awkward
By default it hides your window manager frame so there is only basic window management features provided(what it says in the setting).
But you have an option to show the system's window manager frame if you want.
Con No Dark mode
And not a lot of customization available.