When comparing Arc vs DarK, the Slant community recommends Arc for most people. In the question“What are the best gtk3 themes?” Arc is ranked 2nd while DarK is ranked 8th. The most important reason people chose Arc is:
Users will be able to match their gtk2 and gtk3 themes easily due to Arc supporting both.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
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 Really clean and nice-looking
Pro Supports gnome-shell
Pro Customizable to dark or light themes
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 Actively maintained
Actively maintained, support for it won't be going anywhere anytime soon.
Pro Fully supports GTK3-nocsd
The theme fully supports the GTK3-nocsd hack so GNOME3-apps will integrate well into XFCE or MATE.
Pro The color can be easily changed
A script is included to change the highlight-color to almost everything.
Pro GTK2 and GTK3
Supports both GTK2 and GTK3.
Pro Has full HiDPI support
The theme and all included sub-themes have full HiDPI support.
Cons
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.
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 Close button is not activated when cursor is moved to the top edge of the screen
The simplest of all ergonomic design principles is not followed.

Con Seems to not be developed anymore
Link is not working and it is not possible to find an OnyX theme for GTK3 anywhere.
Con No-GNOME etc. support
As it is made to support traditional desktop environments so there is no support for GNOME3 and its forks
Con Dark theme
It's a dark theme, so you might encounter the same issues as with all dark themes
