My 2c in the interests of helping others ;)
I have used both extensively: Logic is definitely up to a point more CPU friendly but Cubase uses 32bit freeze files by default, and *call me crazy...* but I consistently feel that Cubase sounds better. I just find Logic to have a bit of a tinny sound compared to Cubase. Why I used Logic for many years was because Cubase barely worked on Mac, but I'm happy to say version 6 (latest updates) is worked as solid if not more solidly than Logic is, which is fantastic. Interface-wise, I much prefer Cubase. You may get more performance out of Logic at first glance, but in fact what I have found in the real world is that especially with larger projects or anytime you really push a machine especially multi-core machines with multiple hosts (aka VEPro instances) - Cubase comes out on top.
On the other side, the Groove Quantization set up in Logic is better - you can create templates that are stored with the project, whereas Cubase seems to bizarrely make them global across all projects. So you have grooves from other projects in there - why anybody would want that (If I am doing something wrong and you are reading this and know what it is, please let me know!). In fact this is my only gripe with Cubase vs Logic. Not a deal breaker.
I'm using Cubase in 64bit with OS 10.6.8 (not bothering with Lion) by the way, and have loaded RAM intensive projects with Vienna Software as well as many other instruments from various companies at once, sometimes inside Cubase, and sometimes inside Vienna Ensemble Pro. I have not used it much with Vienna Instruments Pro directly inside Cubase because I have instead used Vienna Ensemble Pro (4 and 5) as the host of course, and Vienna suite both in VE Pro and in Cubase. Reliability is great across the board - I rarely have crashes and recovery is great (and I believe I have always been able to track them to a certain third party plug in that I will not name here but which has not been very reliable) - I highly recommend keeping a save of your VE Pro metaframe and project files, as well as of course saving your host file, in this case Cubase, and keep a backup going a la time machine - your chances of recovery are very high. I haven't had any crashes I couldn't recover from thank God... not so in the past though: Cubase (circa version 4) was abominable and un-useable in any professional or creative sense for that matter (for me).
Cubase has one fantastic feature that Logic doesn't have - when you freeze tracks you have the option to "unload instrument from memory" - you can freeze the track and then go and delete the VEPro instance from memory - you can't do that in Logic. Multi channel functions from Cubase are FAR easier and more intuitive. Very easy to add or delete a stereo stem from Vienna Pro. I run mostly Stereo subs from VEPro instances, but it's always easy to siphon off a solo part or other instrument that you want to treat or mix differently (especially useful when freezing and you find you want to raise or lower a specific solo part - no need to unfreeze the whole instance).
The use of MIDI channels, "lanes" which is really virtual midi channels on a single channel and folders make Cubase really great for organising especially with large projects. You can take a VE Pro instance, 10 MIDI channels and it's output and put them all in a folder, and use that as your "brass" or "woodwinds" section. When you aren't working on that section, you just fold them away. Sure, you can do this in Logic, but it's not the same, not as easy or straight forward IMO. I also think the built in EQ sounds better in Cubase than Logic, but I almost always use Vienna EQ anyway. But for simple things or if I'm in a hurry I know I can reach for the built in EQ and not fear it will sound bad. Logics EQ is ok, don't get me wrong, but if I had to use either Logic's EQ or Cubases EQ across an entire mix, I know which I would go for.
Cubase lets you bounce to 32bit audio files and freeze files are automatically 32bit. (this is nothing to do with the app running in 32bit or 64bit memory space).
Logic has very clever bounce features - you can bounce in several formats all at once in a single pass, with Cubase you have to manually do them one by one if you want to make mp3's etc. But, this is certainly a very minor issue and not a deal breaker. The ease of use, reliability, and great functionality with VE Pro, plus the sound (for me) make Cubase the DAW of choice these days.
Well that's my non-linear review of Cubase ;) Hope it helps you get a feel for it. The things that are most important to me and probably to most others as well are:
- Sound, Reliability, Compatibility, Workflow.
For Vienna Users, IMO, Cubase wins hands down right now.
BTW I haven't used Nuendo, but from what I hear Cubase and Nuendo are cut from the same cloth, so it's basically the same thing.