Recently, Microsoft also provided me a Surface Pro (though this one is very much a work asset). Comparing the two, what I realized is that the raw specifications really don't convey the real-world experiential differences between these two devices. So, I figured I'd share what I've observed so far.
Form FactorThe RT is a slim device. It's not 'light', but the aluminum chassis feels right - solid and rugged without being uncomfortable to use. I've used lighter tablets, and they feel cheap and plastic-y and scream in protest at the slightest torsion.
The Pro is noticeably thicker and heavier - you can tell the difference, especially after using both in succession, but in practical terms, neither is heavy enough for the weight to really matter. Both Surface models are at their best on a table where you can leverage the keyboard and kickstand anyways.
Unlike the RT, there is a small gap along the whole back edge of the Pro, presumably for the active cooling. It's not really a problem, but it's unusual enough to attract attention.
PowerThe RT runs forever. It'll go several days without a charge, and even with heavy use, it'll make it to the charger at the end of the day with power to spare. While the Pro's battery life has been often maligned, it's not horrible for casual use. I haven't tried watching movies yet, but I can check mail and take notes throughout the day at work and still have (a sliver) of battery left when I leave the office. Unlike every laptop I've ever used, I haven't had to compulsively hunt for power everywhere I go.
The RT may warm up slightly if you use it heavily. By comparison, the Pro is cool when idle, but can get quite hot when it's under intensive use, particularly for games. It seems that the Pro also has some active cooling when it's running hot, and it can be surprising when one hears a fan whir up on a tablet.
Sleep is a bit different between the two devices. The RT is "Instant On"; you push the button, and you light up immediately. The Pro is the same when it has power, but if you leave it idle on battery for awhile it goes into a more traditional laptop-style sleep mode. This just means you get the Surface logo and it takes about five seconds to wake up. I assume, much like a normal laptop, the settings for sleep are configurable.
One thing to note - though they are the same plug (and presumably "compatible"), the RT and the Pro's chargers are different. I'm told the Pro's is much beefier, to meet the Microsoft guidelines for device charging times on the bigger battery.
Pen InputOne obvious bonus to the Surface Pro - a full tablet stylus. This isn't a glorified pointy stick that just replaces one of your smudgy fingers, but rather makes the Surface Pro a full drawing tablet with pressure sensitivity, hover, eraser, and pixel-precise resolution. Gabe speaks at length as to the quality of the stylus for drawing purposes. For the non-artistic population, honestly you won't use it much - apps are designed for finger-poking so the precision of the pen is redundant. However, there's one killer feature for the pen - digitally signing documents. The built-in PDF reader will allow you to draw and save, which means you can sign emailed documents without them ever hitting paper.
The pen attaches magnetically to the power socket so you can carry it around. While this seemed brilliant when I first heard about it, it's actually quite annoying because when you connect to power, you end up having to just leave the pen loose. Then the cat knocks it off your desk.
Device SupportBoth the RT and the Pro have a USB port, putting both devices miles beyond Apple-like tablets with their device-specific plugs. The RT in theory only supports a small set of devices designed for it, but it's actually surprising how many USB devices work automatically. I've tried with a few USB keys, a camera, and even my smartcard reader, and they all work! I've heard that most mice and keyboards work magically too.
The Pro is like any other computer - if it's USB, it'll work. Period.
The RT and the Pro both have video out; micro HDMI for the RT, and Mini DisplayPort for the Pro. I have yet to try the Pro, but the RT can drive some devices to high resolution (more than the device itself). Don't buy the expensive adaptors from Microsoft; generic adaptors can be had on Amazon for a few dollars that will work just as well.
Aside: Type vs Touch
The debate rages on. The touch keyboards are thinner, quieter, and come in several colors. You can certainly type faster than an on-screen keyboard, but it'd still be a bit awkward if you're a proficient touch typist and need to write an essay. The type keyboards are black, thick, and undeniably click-y. Click-y is good.
After using both; the choice is obvious. If you're doing a lot of typing, get the type keyboard.
The touch keyboard is good as a cover, and still a big improvement over an on-screen keyboard if you occasionally need to poke more than a text message out.