Nuendo is behind Cubase in some features, and then again Cubase is behind Nuendo in others. So it’s a leapfrogging kind of thing. Right now, Cubase 10.5 got some great new features that Nuendo will get in the Spring. However, Cubase just got some features in 10.5 that Nuendo had, such as video export. So it’s not really true that one is really ahead of the other. Just depends on what features you are looking at. And ultimately, Nuendo will get everything that Cubase has, but Cubase will NOT get everything that Nuendo has. So if you want the FULL COMPLETE Steinberg DAW feature set, then you go with Nuendo, which will eventually have everything. HOWEVER, having said that, the features that Cubase will NOT get are related to more advanced post production and game features so in that sense, it may not make any difference, and it may make more sense to stick with Cubase.
As for Cubase being the beta for Nuendo, I don’t think that’s true. It seems that way sometimes, but it’s the result of this leapfrogging situation. It’s really just with the NEW features when they are out of alignment. It can be said that Nuendo is the beta for Cubase on the NEW Nuendo-related features that Cubase just got, like video export, etc. So it depends on what you are looking at again.
In the end, you should just stick with the DAW that makes the most sense for YOU. If you don’t plan on using the more advanced post features of Nuendo, and you are ONLY doing music, then just stick with Cubase. And like @trmupstage just said, use 10 until the 10.5 bugs are worked out.