I have Nuendo 6 and Cubase 7.5. I do mainly music with the odd music to picture job. I much prefer the GUI of Cubase now as it’s more editable than nuendo. For example the waveforms in Nuendo can’t ne coloured the same as Cubase, the highlighting in Nuendo puts a red line around the part, I prefer the Cubase way now where the whole part changes colour.
In the old days I much preferred the GUI of Nuendo but since the latest incarnations I very rarely open Nuendo these days.
Projects open fine between the two programs so don’t worry about that.
MC