When comparing Orion vs Arc, the Slant community recommends Arc for most people. In the question“What are the best gtk3 themes?” Arc is ranked 2nd while Orion is ranked 21st.
Ranked in these QuestionsQuestion Ranking
Pro Plug-ins including Pixlr image editor
Pro Built for front-end development
Orion was developed specifically for front-end development, making it a strong option for front-end developers thanks to features like syntax validation with ESLint and HTML content assist.
Pro Eclipse-like UI
Orion is developed by Eclipse and shares a lot in common with the Eclipse IDE UI.
Pro Customizable to dark or light themes
Pro Has both GTK2 and GTK 3 support
Users will be able to match their gtk2 and gtk3 themes easily due to Arc supporting both.
Pro Available firefox and chromium themes
There are matching themes for Firefox and Chromium for those that want to have a fully matched looking environment.
Pro Supports gnome-shell
Pro Really clean and nice-looking
Con No runtime environment
Does mostly do file editing, but without good project support, web terminal, build/runtime environment.
Con Bleak font colors that cause eye strain
The font color in Arc and Arc-Darker is not black, but lightgray
after prolonged usage, it becomes apparent that this font color, combined with white backgrounds causes too much eye strain
Con Has some contrast issues
The theme is a low contrast theme so it really depends on the screen brightness if you see borders or the text, some elements don't even have borders - this makes the theme barely usable for portable devices.