I'de agree with Zeno's timeframe.
That give's people time to port their customisation to the new theme without adding too much work for the developers in coding for three theme's simultaneously. We, at PTFS Europe have been enabling CCSR by default for new installs, and often 'upgrading' to CCSR during koha upgrades. Not everyone is going to suddenly be upgraded at the same time, so it shouldn't be too harsh a problem to take OPAC customisations into account during an upgrade.
As for Paul's comments, Robin has pretty much covered it. Bootstrap is a framework of well supported JS and CSS that we could not hope to create and maintain for ourselves, not using one of these frameworks (Bootstrap, Foundation) in the foreseeable future is just plain silly given the work it takes to create fully responsive themes the meet current expectations. Using bootstrap is much more likely to pass tests than us reinventing the wheel.
So.. all in all the bootstrap theme gets my vote!