I just installed bootcamp/Windows 7 x64 under Lion (in full 64-bit mode) on a 2009 Macbook Pro to help understand this exact question.
This was not a very fair “Cubase” test, as I have way too many 3rd party VSTs – of which any could be culprits – but, a projects that plays back smooth as butter under the Windows 7 version of Cubase (64-bit version), with CPU to spare, won’t even play at all under the OS X version of Cubase (64-bit version). Same hardware.
That’s an unfair and unscientific test, but for my purposes, told the story I cared about with my situation.
Also, you might find this of interest. It’s VERY outdated, and also very dependent on 3rd party plugins and other factors, like buffer size, ASIO vs Core Audio, etc. It also points to performance issues with OS X itself, likely outside of Steinberg’s control. So, you really have to study it and not draw any quick conclusions. That said, it seems to confirm what I’m experiencing with overall poorer performance under OS X with a multi-3rd part VST setup “as a whole.”
(be sure to read parts 1 - 6 of the article)
But, what’s great is that you have the option to choose if you need to. One of the many reasons I just switched from Logic to Cubase.
And Fizbin’s right, you’ll never get a proper, detailed response or inspired acknowledgement from Steinberg on this, just a canned reply. I asked the question and the response I got was something along the lines of: “some say it works better on Mac, some say it works better on PC. both versions are 64-bit. depends on your hardware and ram, 3rd party plugins, etc.”
All that said, I’m still actually using it under OS X. Ha!