The problem I see with using Parallels on a Fusion drive iMac is that Parallels will install on the 128GB SSD, and will then install Windows on that SSD. The default space Parallels 8 allocates for Windows 7 or 8 is 64GB. I know that this can be reduced, but a typical install of Windows with apps could approach 50GB of valuable SSD real estate, although you will have a snappy Windows installation. You could install Windows with very few applications, but what's the point of that? If you value your SSD space, it looks like the best bet for installing Windows is to use Bootcamp, which will partition all the space you need for it on the HDD. It may run a bit slower, but the SSD will be saved for OS X.