Well first of all PC/Mac fanboyism runs deep. I wouldn’t necessarily trust that every person even on the COW is completely unbiased. I have nearly the same exact iMac you do, and I can tell you that it performs pretty well indeed. Yes, it can get a bit slower if you are using a lot of 3D effects and optical flares, but that is to be expected.
As far as PC/Mac goes, I have used both fairly extensively. I can definitely say that while I hate the cult that surrounds Apple, I find OSX to be a smoother, more intuitive and generally less glitchy OS than Windows. (For the sake of specifics, I have used XP and 7 quite a bit, I have not once touched 8). Having said that, Windows can be a fine operating system if you take care of it. It’s important to only install trusted software and not bloatware from random websites that will install all kinds of other nonsense on your machine. If you keep it clean, it should work great. There will be crashes and problems, but that happens on OSX as well. No OS is perfect, despite what the fanboys will tell you. It seems that many production houses are switching from Mac Pros to HP workstations, and while many people were nervous about switching from Mac to PC they have done it with little to no hiccup.
As far as what machine to go with, I have built a couple computers but I am not an expert on current ratings of which processor is faster, et cetera. It seems to me that the compact PC you list is about equal in power to the laptop, and with much less portability.