When comparing viva vs Zukitwo, the Slant community recommends Zukitwo for most people. In the question“What are the best Gnome Shell themes?” Zukitwo is ranked 6th while viva is ranked 12th. The most important reason people chose Zukitwo is:
Zukitwo theme offers a matching GTK theme to the built in Gnome Shell theme.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Available in Blue and Orange themes
viva offers a blue and an orange them so users can pick how the theme looks.
Pro Built in matching GTK theme
Zukitwo theme offers a matching GTK theme to the built in Gnome Shell theme.
Pro Offers PPA for easy install on Ubuntu based distros
Zukitwo offers an Ubuntu PPA for Ubuntu based distros.
sudo add-apt-repository ppa:noobslab/themes
sudo apt-get update
sudo apt-get install zukitwo
Cons
Con Not finished
The them is not 100% finished yet, though the author does say he is working on it. Of course the latest commits can be seen on the github page with the last being from Sept. 2014.
Con Only supports latest version of Gnome Shell and GTK
Zukitwo only officially supports the latest release of Gnome Shell and GTK.