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I use Fusion but thought i'd give virtualbox ago, my big con you cant run it off a windows partition so bang goes native speed should you ever need it ( in my case i do so i fogged it off out of it ),never used Q Emulator so cant comment on that
 
I use VirtualBox with Windows 2000. No problems so far, it's stable and fast but not polished as Parallels or Fusion.
 
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