I actually think market share and user base are important factors when deciding on a system. When you work with a computer as much as I do, your most important technical support comes from other users. So the quality and size of the user base is an important topic and very underrated when considering a system.
In terms of editing, FCP has a significant enough user base to handily beat Premiere. Avid is still used by more pros, but FCP is rapidly overtaking the indie market. And THAT, to me, is what gives a Mac the edge in editing, not the OS.
In terms of features, FCP and Premiere are virtually indentical. In my experience, FCP is not quite as stable as Premiere, but it does do other things a lot better, like 24p support in QT. I regard those things as trade offs, however, and am moving to FCP thanks to the ubiquity of FCP around me in the film industry and because I can't afford a "real" (ie. turnkey) Avid system.
That said, while FCP does have a significant user base, which I appreciate, I am really turned off by a lof of the Mac user base in general. It's one of the reasons I haven't and won't completely switch to Macs altogether. It's a real turn off for all the reasons mentioned earlier.