If you could choose freely, what processing platform for still images would you choose?
The one you feel more comfortable with.
From an hardware perspective, since Apple moved to the Intel platform, there's very little difference from an high-end "Windows" PC and an "Apple" one (but maybe the "stylish" part).
With Windows you may have more choice, especially if you're ready to build your own PC - but it also means warranty covers the single parts, not the whole system - and you need to be careful to get and match the proper components. Gaming-oriented ones, although fast, may not be the best choice for image processing where you also need stability and minimal data corruption risks.
I prefer to perform most work an a desktop system. It allows for faster CPUs, more RAM, SSD + RAID (fast) spinning disks, and a good GPU. Photoshop and Lightroom of course work better with a faster CPU and GPU - and good RAM amounts - but it's important to find a "balance point" within your budget, spending too much on some components and too little on others may not lead to a good system. Good power supply and cooling are also important for error-free operations.
Matched with a good monitor, preferably a wide gamut one with hardware calibration, and good input devices (keyboard and mouse - maybe a graphic tablet) it's a comfortable system to use even for long processing sessions.
All-in-ones IMHO are nice devices when there is little space for a classic "desktop" units, laptops are not a choice and style is important (no cables clutter, etc), but they force you to buy a "bundle" like laptops and suffer from the same limitations when it comes to add or expand hardware, and are not often designed with specific tasks like image processing in mind. I would avoid them.
I found laptops less comfortable to use, even with a docking station and a separate monitor. SSDs put an end to slow laptops disks, but they may be not very large (but slower, hybrid ones) and make you depend on external disks or NAS units even for "main" storage. Also, when running heavy tasks and getting "hot", they can become noisy.
Unlike some desktops with enough RAM slots where you can add easily more RAM as you need, and prolong system life a bit, often laptops are not easily upgradeable. Of course for on-field and travel use there are no other choices, although some of the latest tablet/laptop combo may be an interesting alternative - some high end Windows model can run PS/LR although they costs as much as a high-end laptop.
The OS is really a matter of what you are more comfortable with. As a long time Apple user you may not like Windows too much - and PS/LR are quite the same on both platforms. Unless the hardware choice don't force you to switch, I'd keep using it - in some areas like color management it's better and easier to use than Windows. If you keep your system "clean" and mostly dedicated for image processing as you would do with a Mac - avoiding to install all the crap many Windows users ends up to have on their system, IMHO Windows is not less stable or slow than OSX, of course you'd need to be more careful because most malware targets Windows.
Even if many suggest to avoid it, Windows 8.1 has some more advanced features than 7, especially when it comes to recent ones like high DPI support (which Adobe will support under Windows too), improved graphic support, better support for recent hardware (i.e. native USB 3.0 and UAS support), and a good, built-in hypervisor for VMs. Sure, it has the non-desktop-friendly UI - but when you spend most of your time in a couple of application only - like when using LR + PS - it doesn't get much in the way.