There's honestly nothing unique in Nuendo specifically for "production and mixing of music (MIDI played by VIs)" that Cubase doesn't already have, unless you want to explore more surround capabilities plus more post/gaming aspects of music production... but even then, if you're staying purely in the music production realm, it's not easy to justify. However, if there's even a chance you will be exploring those post- or game areas, then now's the time to crossgrade. Not sure I've ever seen the crossgrade price this low. To me, it's a no-brainer to move over if those extra features mean something to you. That's really something only you can answer. Trust your gut on this... will you actually *use* those extra features?
Nuendo is of course for post and game workflows, at which it excels. Of course it can do music production just like Cubase, but the extra features and cost (and increased ongoing upgrade fees too, don't forget!) are most likely not worth it for music-only workflows TBH. The crossgrade money can probably be better spent on great third-party tools/plugins. And honestly, if you're still on Cubase 9.5, you'll get more out of Cubase 10 than moving over to Nuendo for music workflows, since Cubase 10 introduced a bunch of outstanding features that make it a no-brainer upgrade IMO! All of those new Cubase 10 features will show up in Nuendo 10 of course. But the question is... why haven't you already upgraded to Cubase 10? If you hesitated because you were unimpressed with Cubase 10's new features, then Nuendo will not impress you either, other than for your stated surround needs... in which case, Nuendo is good.
Having said that, there are some nice enhancements in Nuendo 8.3 compared to Cubase 9.5 that *might* have some small bearing on your music workflows... off the top of my head, the things I like are... a) the improved Direct Offline Processing (which BTW, dang, is now in Cubase 10, so don't move to Nuendo just for that!), b) the audio alignment features (oh, BTW, those have been added to Cubase 10 already too in a new Audio Alignment Panel), c) AAF support (oh, dang, that just showed up in Cubase 10 too!), d) editing to picture mode (oh yeah, that just showed up in Cubase 10!)...
Okay, I have to really think here... Cubase 10 actually picked up a number of cool features already from Nuendo... maybe Cubase 10 is the one you should move to??? Oh, there's the Sound Randomizer which is kind of cool... it's used in Nuendo for sound design purposes... very helpful, might be usable for music workflows too... and that has NOT been added to Cubase 10, lol! Yet.
Honestly, there's nothing I can think of that Nuendo does that would significantly justify the crossgrade price (and continued INCREASED update prices over the years) for purely MUSIC workflows.
On the other hand, we don't know what the future holds for Nuendo 10, 10.5, 11, and without seeing the list of features for Nuendo 10 yet, I wouldn't be surprised to see some nice post production features that YES, would indeed be helpful for music, that will be exclusive to Nuendo. I'll give the example of Direct Offline Processing, which was always one step better in Nuendo, until now... Cubase 10 finally caught up in DOP functionality. However, perhaps Nuendo 10 will once again leapfrog over Cubase 10 and get a better version of DOP again. Who knows? Things like that might justify moving over to Nuendo for music since IMO, DOP is such a great tool for music production too, not just for post production and sound design. So there are some post-related features that do have relevance for music producers, and only time will tell which ones Steinberg will make exclusive for Nuendo. So if you don't like to be without the best version of a specific Steinberg feature, then go with Nuendo, since it will have everything Cubase 10 has, plus more.
Anyway, my view is to suggest to wait a bit to see what Steinberg announces... that might make a difference. You have plenty of time to decide. The special promotional crossgrade price is supposedly going to be good until Nuendo 10 launches...
As for me and my workflows (which deal with music for film/video and post production), all I can say is that I see no need to keep Cubase around anymore. Timo has stated they are trying to close the gap between Cubase and Nuendo development calendars now and so it doesn't seem that those of us who maintain both licenses need to keep Cubase. I can finally get rid of my extra Cubase license, or better yet, crossgrade my Cubase license to get another Nuendo license.