When comparing Arc vs Moka, the Slant community recommends Arc for most people. In the question“What are the best Gnome Shell themes?” Arc is ranked 1st while Moka is ranked 3rd.
Ranked in these QuestionsQuestion Ranking
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 Supports gnome-shell
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 Really clean and nice-looking
Pro Several distro specific instruction to download
Moka offers suers easy to follow instructions to download including PPA's for Ubuntu based distros and Fedora specific instructions to install its repo.
Pro Wide support
Besides this Gnome Shell theme there are multiple GTK and icon themes that match, giving the user a wide variety of themes to mix and match that still look great together.
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.
Con Does not work on older version of Gnome Shell
Moka does not work on Ubuntu 12.04 nor will it work on Debian Wheezy as they both use old version of Gnome Shell.