I am a huge fan of the iPad, so I don't make that claim lightly. I have owned all three iterations of the iPad thus far, and I'm not prepared to abandon the New iPad any time soon. That said, I also love Windows, and I appreciate that businesses that rely on Windows PCs and Windows-based applications would love to have something like an iPad that also happens to be Windows.
As it happens, I'm also in a position to compare the two side by side...almost. I don't have a Surface tablet, but I do have a running the latest Windows 8 Release Preview. As far as devices actually available today to the general public go, this is about as close as you can get to what you can expect a Windows 8 tablet like the Microsoft Surface to be.
On paper, the Series 7 Slate PC is thicker, larger, and heavier than the iPad. The iPad (Wi-Fi only model) is only 9.5 inches long, 0.37 inches thick, and weighs a mere 1.44 pounds. The Series 7 Slate PC, on the other hand, is 11.66 inches long, 0.51 inches thick, and weighs nearly two pounds. The Windows 8 Pro model of the Surface will be very to the Series 7 Slate PC.
To some extent, though, that's really a matter of splitting hairs. If you stop to consider the fact that the Windows tablet is packing an Intel Core i5 processor, 4GB of RAM, and a 128GB SSD, it's actually fairly impressive that it's as svelte as it is. It also has USB and micro-HDMI ports, and a micro-SD card expansion slot. Plus, it has a trump card the iPad lacks--it runs Windows.
Really, it would make as much or more sense to compare the Windows tablet against existing Windows desktops and laptops. Yes, it's a tablet, so you can't avoid the comparisons to the iPad. However, we're talking about a device that can replace the PC completely rather than a mobile device that augments the PC. I assure you the Series 7 Slate PC is significantly thinner, smaller, and lighter than my Dell XPS M1330 laptop.