I'm in the camp that believes version 7 is the initial version shipping on Model X. It runs "natively" on the CPU/graphics hardware that ships with Model X, and has a platform conversion to Model S, which features older hardware. We may see more lag and/or jerkier animations & scrolling on Model S.
We may see the platform conversion first, if Model X suffers more delays and they need to keep feeding the appetites of Model S owners, though this would be suboptimal, as Tesla would want customers to use the native platform version for reference.
I've long wished for an upgrade package to become available so that older Model S cars can be upgraded to the new dashboard and touchscreen systems. For a pretty penny of course... it's Tesla. It would be a win-win situation, as Tesla would have less of the older systems to support.