Nuendo is different from Cubase. For starters, Cubase does not have the same crossfade editor that Nuendo has. Nuendo's crossfade editor is not as good as Sonic Solutions or Sequoia, but it's a lot better than PT.
The sound quality is pointless to argue about in general except in one regard. As a solid Nuendo user, I feel that the summing is probably a shade better in PTHD. I have the RME TotalMix mixer which is 40bit linear and the summing is a hair better than native Nuendo so it's reasonable to assume that a 48bit linear system would also sound a little better. These are very small differences, however the user interface still makes me lean toward to Nuendo (not to mention that i have a 40bit linear summing solution that was free with my converter so I don't need it in PT).
The external FX delay compensation in Nuendo is awesome. I really love it, it works like a charm. I can digitally patch to a Lexicon unit that sounds better than any reverb plug-in I've used and get full delay compensation. Not to mention analog comps. I know PT can do external inserts, but I'm not sure how it deals with the delay.
The automation in Nuendo is lacking, but it's not that much better in PT. Neither has snapshots. The best automation I've seen on paper is Samp/Sequoia - haven't tried it yet tho. Nuendo only recently fixed a major problem with the Trimming feature, but it is fixed now (since 3.1). At this point there's not much difference between them in that regard.
A major factor when comparing these two programs is that Nuendo has full-featured MIDI. PT MIDI is.....well..for all professional intents and purposes PT doesn't really have MIDI. Nuendo conversely has everything Cubase has in that department. Another huge plus for Nuendo is the Post Production features. I don't do this work so I'm not as familiar with it as some, but for the time that Pinnacle owned Steinberg, Nuendo got a boatload of Post features. I really don't think PT competes anymore in this arena (except for plain inertia or not wanting to upset clients). Feature-wise tho Nuendo is king of this realm.
I happen to think Nuendo has far more advanced user interface. Like anything that offers greater flexibility, it's only useful if you really take the time to learn it. I agree that for a new user, PT is more accessible and easier to learn. But as you start reaching for features and "wishing you could do" this or that, Nuendo is much more satisfying. Nuendo is more about "How do you want to work" rather than "This is how you need to work". Yes, the learning curve is steeper, but that's always the case with more features. If you can learn Logic or DP, you can learn Nuendo just fine. My feeling is that PT is simple to a fault.
BTW, there were some statements that the celeb list of nuendo users have people running it for them. Not true. Elliot Schiener just did a guest stint on the Nuendo forum. He definitely runs his own rig and so does Ed Chereny and most of those people. Nuendo was created specifically with their input and desires. They were the orginial "Producers Group" that Steinberg turned to.
For Nuendo users, all you need to do is keep an Mbox or 002R around for converting projects from the ubiquotous PT format. Including that expense, a balls out Nuendo rig is still 1/2 the price of a PTHD rig. Also the most recent Nuendo upgrade includes AAF conversion which is supposed to be able to deal with PT files. Haven't tried it yet.
BTW, in addition to Eventide we also miss out on Massenberg Plugs. C'mon George! We miss ya!