It's a stormy Saturday afternoon and I have time to do this, so settle back for a long one...
I'm on my second iPad (do we call it "3G" now?), and I have a Samsung Series 7 Windows Slate (last year's model). The Samsung was Microsoft's reference platform when they were building Windows 8, so if any pre-RT tablet runs Win8 well, this one should, and does. So I have long experience with both platforms, as tablets.
First, I need to distinguish between Windows RT and Windows 8, because this will be a confusion factor for a long time to come. At a hardware level, "Windows RT" refers to ARM-based tablets, and "Windows 8" refers to Intel-based tablets. At an OS level, they're the same operating system, except where differentiated between consumer and enterprise versions. At an application level, the new programming interface is "WinRT" regardless of platform. WinRT applications ("Windows Store Apps", formerly known as Metro style), run on both ARM and Intel platforms. The operating system also supports a "classic" desktop mode; on Intel hardware, this will run anything that runs on Windows 7. On ARM hardware, the desktop runs only MS Office and IE 10; MS plans to keep this locked down. Clear enough? Probably not....
Oh yes, IE 10 runs in both modes, as desktop and as "Metro style". They look and feel like completely different browsers, and follow the appropriate rules and guidelines for each environment.
Because Windows tablets are being made by numerous vendors in both ARM and Intel configurations, comparisons to the iPad are not simple. The high-end Windows tablet hardware will beat the socks of the iPad (and will be priced accordingly); the low-end tablets will be inferior to the iPad. You can only compare specific models. However, you can expect most or all Windows tablets to support keyboard, mouse, Bluetooth, USB, HD cards, HDMI out, and through docks, Ethernet and external monitors.
So on to comparisons:
Operating system: Both iOS and Windows 8 have discoverability issues, but the learning curves are roughly equivalent. Windows 8 is very smooth and fluid, and the start page scrolls continuously, unlike iOS with multiple "pages". While the live tiles have been called an "ADD interface" (as in attention deficit disorder), they do present a lot of information about things you might want to know, without opening any apps. What's on my calendar? Who's emailed me, and what about? What's topping the news? What's the temperature outside? Organizing the tiles is also pretty flexible. Settings and search are easily accessible without leaving the start page, simply by swiping in from the right edge. You can switch applications by swiping in from the left edge. Application functions are accessible by swiping in from the top or bottom edge. Application search, share, and settings use the same "swipe from right" and tools that the OS provides. (Microsoft enforces this when allowing apps into the app store.) Overall, Windows 8 wins over iOS here; it's fast, fluid, highly consistent, and stays out of your way. The live tiles are a big step forward. The "snap" feature that allows two "Metro style" apps to display at once is also a big win.
Another OS consideration is that Windows 8 exposes the file system, while iOS pointedly does not. Document sharing between apps and between individuals is a lot easier on Windows.
Cloud usage: I'll rate them equal. iOS and OSX share settings through iCloud. Windows 8 does the same across devices and PCs. iCloud stores files. So does SkyDrive, but it's available for Windows, OSX, and iOS, which is nice.
Apps: On launch day, Microsoft had over 5000 apps in the Windows Store, not bad for a new platform. The store rules are similar to Apple's app store, with a fairly strict approval process. While you won't find all your favorite apps there yet, I expect the app store to grow quickly. The APIs and programming tools are an evolution on Microsoft's already excellent tooling, and there are many millions of developers who can make that transition quickly. Many already have. So (for Windows RT, at least) iOS wins for now, but this will evolve.
For Intel-based tablets, though, the situation is much different. My Samsung tablet runs Photoshop CS6 under Windows 8. It runs Visual Studio for software development. It can run web and database servers (real-world production stuff, like SQL Server and IIS). My iPad doesn't do any of these things.
Business use: Windows has a big advantage here, simply by being Windows. Existing business apps will run on Intel-based tablets. Windows 8 supports all the system management and security tools that enterprises demand. While business will generally not move quickly (they never do), I expect Windows 8 to work for BYOD scenarios much more easily than iOS or Android.
Consumer use: Here's there the iPad wins. The retina displays are outstanding, and the 4:3 form factor is easier to hold. While high-end Windows tablets will have 1080 HD displays, those won't be in the iPad's price range, at least for a while. For media consumption, I'll stay with my iPad; I read books (Kindle app), surf the web, watch videos. Windows does all these things, and also does them well, and I could be happy with just my Windows tablet. But my Samsung tablet is a bit heavier, doesn't have the same battery life, and the Kindle app isn't quite as smooth in paging (but it's getting better). I do like the wide aspect ratio for video, though, and hopefully the power management will improve with maturity both in hardware and software.
Content creation: I'll leave this one wide open. It all depends on what you want to create, and what tools you need, both hardware and software. Laptops and desktops are going to rule in this area for a while longer, both for Windows and Mac.
Marketing: Here's the rub. While Intel-based Windows 8 tablets are clearly oriented to business and professional users, I really don't know where the ARM-based Windows RT is going. By contrast, Apple's marketing is pretty clear, with the only drawback being that the crossover between iOS and OSX isn't so easy.
So there it is, one person's opinion.