Hi,
Differences between Cubase and Nuendo is like ProTools and ProTools HD. Same software with extended features.
Actually the real problem with Steinberg is that with each new version of Cubase / Nuendo you have bug fixes but new bugs coming. You have to wait for the new version to have the bugs solved but expect to get new ones in the meantime. It’s a pretty disturbing thing in a professional setting.
Anyway, if you write music for films, Nuendo is a good choice.