Disclaimer: I have not really pushed the limits of video editing. I am somewhat of a tech geek, and have asked CS6 to do some 3d renders and high def video, but always playing around.
I've had a really great experience with my 27" iMac. It's a late 2010 model, 2.93 GHz 4core, 12GB RAM, Radeon 5750 w/ 1GB. CS6 works great, although sometimes I come close to bumping up against the RAM I have- so I might recommend 16GB or more. But as far as your current workflow, I think the iMac would do nicely. Two big problems with the macbook pro: screen size, and heat. A friend of mine has the retina pro, and it gets quite hot when working hard. The larger enclosure of the desktop helps get around this. There's also absolutely something to be said for that 27" display: I have both the iMac and a non-retina MBP, and 15" never felt small until I had the desktop.
As far as the PC option, I have to say, I don't know nearly as much, although performance specs are performance specs. If you can get a new i7 processor, at least 16GB of RAM, and a good video card and display, I imagine you'd be happy. Ultimately, that's a question that a lot of people will weigh in on, and you have to decide on for yourself. Would you rather try final cut, or PC-only-3D software? I am a very happy mac user, and would recommend the platform to a lot of people, but there are reasons to go PC, and you just have to weigh the pros and cons of each for yourself.