To look at them, there's no much difference between the HP TouchPad, Motorola's Xoom (arriving later this month) and the Apple iPad. But if you look beyond the roughly 10-inch touchscreens, you'll find some significant differences.

The TouchPad rocks nearly identical physical size and weight as the iPad and the screen size (9.7-inch) and resolution (1024 by 768 pixels) match the iPad's specs too. The Motorola Xoom has a slightly larger screen (10.1-inch) and resolution (1280 by 800 pixels), making it slightly larger, though just as heavy. Together, the three tablets represent the top end of modern tablet computers.

Hardware

The TouchPad runs on a speedy dual-core Snapdragon processor from Qualcomm. The Xoom is also dual-core, with an NVIDIA Tegra 2 chip. Only the iPad runs on a single core Apple-customised A4 processor. The iPad also has the least amount of RAM (256MB), while the TouchPad and Xoom run on four time more memory (1GB). The faster processors should help the tablets open and run apps faster, while more RAM aids multitasking (both improvements take a toll on battery life, though).

The TouchPad will come in 16GB and 32GB versions, while the Xoom will come in just one 32GB version. The iPad also has a 64GB version. Only the Xoom offers expandable storage via an SD card slot.

The TouchPad will initially support only Wi-Fi connections, with a 3G version to come later. Motorola is taking the opposite approach with the Xoom, which will initially launch only in a 3G version, with a Wi-Fi-only version to follow. The iPad offers both WiFi and 3G versions.

The TouchPad has only a front-facing camera for video calling. The Xoom will have two cameras, one on the back and one on the front, and the iPad has none. The iPad also lacks stereo speakers and a gyroscope.

Other perks from the iPad competitors: the TouchPad can charge wirelessly with a separate accessory. The Xoom can output HD video.

The biggest differentiator between the tablets will be their different OSes.

Software

The HP TouchPad will be the first tablet running WebOS, the operating system HP acquired by purchasing Palm last year. The TouchPad will employ the same card stack metaphor for organising apps found on Palm smartphones.

That interface looks even more at home on the larger screen of a tablet. There are few details about changes to the OS for the tablet right now and reviewers got no hands-on time with it.

Android 3.0 ("Honeycomb") on the Motorola Xoom is also new. The OS was specifically designed for tablets, and brings several new interface metaphors, previously unseen on Android smartphones (see an in-depth tour of Honeycomb).

Lastly, iOS on the iPad (due for an upgrade soon) is familiar to anyone who's used an iPhone or iPod touch.

Because of its massive head start, the iPad has the most apps available for it (over 60,000). Presumably, only a small selection of programs will be available for the TouchPad and Xoom at their launch. Of course, both will likely be able to run apps designed for WebOS or Android smartphones, but experience with the iPad has shown that running smartphone apps on a tablet is generally unsatisfying.

Price

Perhaps the most important unanswered question is how much these new tablets will cost. You can buy an iPad for as little as $499. Early rumours are that the Xoom may start at $799 (ouch!) and HP didn't give any information about price at their Wednesday event.

Finally, let's not forget that the iPad is an year old and it's due for a refresh any time now. This means that by the time the TouchPad and Xoom arrive on the market, they will not only have to fight the old iPad (which may remain on sale at an even lower price), but also a freshly revamped iPad 2.

Which big tablet are you thinking of buying? A TouchPad, a Xoom, an iPad, or will you wait for the iPad 2?