The Driverless 1.6.x is the Long Term Support branch. If you are already developing models using this branch, we would suggest to continue using 1.6.x series as we will continue supporting this branch and 1.6.4 will be the next release in the series coming out sometime early next month.
Where as 1.7.0 is the Latest Stable Driverless AI release with new features added in that are still maturing and will be later merged into the Long Term Support branch. The main attraction of 1.7.0 is Bring your Own recipes , details can be found - https://github.com/h2oai/driverlessai-recipes. Please note that we do not support MOJOs for custom recipes yet.