When comparing Numix vs DarK, the Slant community recommends Numix for most people. In the question“What are the best gtk3 themes?” Numix is ranked 4th while DarK is ranked 8th.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Great for people who prefer dark themes
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 Flat buttons and controls
Con Not free
Official Numix Dark and its wallpapers are not free. The other one with the Xfce-look is free, though.
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