"It's not either/or; you can have it all"
Tiki 12 is a Long Term Support (LTS) version, which is supported for 5 years (vs 3.5 years for 9.x LTS). Among major new features and enhancements include:
As part of our continual "dogfooding" process, the *.tiki.org sites were upgraded to Tiki 12 several weeks before the release, allowing the Tiki Community to obtain "real world" feedback for the upgrade.
Tiki 6.xLTS and 9.xLTS support continues
The support period is as follows:
9.x LTS: Supported until Tiki 16.1 is released (November 2015)
6.x LTS: Supported until Tiki 13.1 is released (May 2014)
Which Version is Right for Me?
The Tiki Version Lifecycle page (http://info.tiki.org/Version+Lifecycle) will help you choose the right version to install.
Work is already well underway on Tiki13. The main change is the move to the Bootstrap mobile first front-end framework. Like any post-LTS version, it will have a lot of enhancements. In fact, current estimates of development planned will make it the release with the most development ever (in terms of developer hours)!