Unless I have missed something, Nuendo 10.0-10.2 is a superset of Cubase Pro 10 and Nuendo 10.3 is a superset of Cubase Pro 10.5 with one exception - Nuendo does not have or support VST Transit.
Other than the omission of VST Transit, which I suspect is not an issue for most people, there are two drawbacks of purchasing Nuendo:
- The Nuendo product cycle lags Cubase - Nuendo has only just got the new features introduced in Cubase Pro 10.5 which was launched at the end of November 2019. Perhaps there is a six month lag, give or take - Cubase gets annual feature updates in late November and maybe Nuendo will get annual feature updates including the most recent Cubase feature updates in late May or early June.
- Paid version updates are likely to be more expensive for Nuendo than for Cubase
Both these are only tentative conclusions, since Nuendo was only aligned (with a lag) to the Cubase product cycle at version 10. From that point, Nuendo has yet to have a paid version update: Steinberg are giving away the Nuendo 10.3 update for free, which is the equivalent of a (chargeable) Cubase 10 to 10.5 update. Nuendo 10.3 is the only Nuendo 10 update that has added significant new functionality beyond that promised in the launch day specification for Nuendo 10. (From memory Nuendo 10.1 added ARA2 support and Nuendo 10.2 added some missing functionality for working with video files.)
If you have any use for the additional functionality in Nuendo, crossgrading to Nuendo is your best choice but with the proviso that you are then locked into the Nuendo product cycle and upgrade costs. You cannot downgrade Nuendo to Cubase and, for reasons I cannot fathom but perhaps have been explained previously, you cannot run Cubase Pro with a licence for the corresponding version of Nuendo.
If you have no need of the post-production and game development features in Nuendo, you might be better off sticking with Cubase.
If it is Cubase Pro that you want, I would try to do a deal with someone who wants to crossgrade from Cubase to Nuendo in the current sale. If I, as a Cubase Pro 10.5 owner, follow through on my interest in crossgrading to Nuendo 10(.3) in the current sale. I would give up a Cubase Pro 10.5 licence for the Nuendo licence. It would make more sense to use a much less valuable Cubase 5 licence to crossgrade instead. The easiest way to do this would be you sending your USB eLicenser with your Cubase 5 licence (and ideally no other licences) to the aspiring Nuendo purchaser. They move the Cubase 5 licence to their USB eLicenser, move their Cubase Pro licence to your USB eLicenser, then disconnect your eLicenser and carry out the crossgrade before returning your USB eLicenser to you.
The financial side of the transaction would be up to the parties, though the cost should be probably based on splitting the cost of the Nuendo crossgrade plus covering shipping and other costs.