The biggest benefit of using Mac OS for video editing, is the ProRes family of codecs. There's simply no (free) equivalent on Windows. ProRes comes "out of the box" and is widely used for production and delivery.
I've used ProRes ever since it was launched, and I'll still use it until broadcasters and clients want something else.
Premiere Pro is basically the same whether you're on Mac or Windows. You get CUDA and OpenCL acceleration on both platforms.
While FCP 7 still works, it really shows it age when it comes to format and codec support. And the fact you can't make timelines bigger than 2K resolution. Not many will need more than 2K anyway, but for those of us who do theatrical releases, it's been a major pita.
Premiere can work with any resolution it seems, and it's very good at handling different codecs and file formats. I only wish they could get timecode embedded on SDI output to work.
As for FCPX, I'm not too familiar with it. Yet. I've used it on a couple of small projects, but that's back when it was brand new and painstakingly slow on my old school Mac Pro. It works a lot better on a computer with newer CPUs (any Mac mini, MacBook, iMac and the latest Mac Pro with i5/i7/last gen XEON CPUs) than it does on a dual CPU Mac Pro 2009! Even the Mac mini I have gave me a better experience of FCPX than my Mac Pro.
I can't say why or how - it's just the way it works.
I really like the timeline of FCPX. The way you're not bound to tracks, but rather it automatically move the clips away if needed and other smart editing tools. I do not like FCPX when it comes to mixing sound, simple color correction or text. It's a major pain to work with.
The color correction tools makes absolutely no sense to me, but then again, if you're not used to dedicated color correction tools you might not even think about it.