Two gadgets that began shipping last week represent assaults from Google on the dominant model of computing, in which we use a cursor and a keyboard to manipulate boxes and windows on a virtual desktop. Samsung makes the hardware for both: the Series 5 Chromebook notebook, the first computer with the browser-only ChromeOS, and the Galaxy Tab 10.1 tablet, whose operating system is the latest version of Honeycomb, the tablet edition of Google’s Android mobile operating system.

These products have arrived at a pivotal moment for computing. Steve Jobs popularized the phrase “post-PC era” to describe what’s supposed to come next, with the iPad displacing the window-driven, desktop-focused experience that the word “computer” conjures up. Now Google too is offering alternatives to that experience, taking on traditional computing with a pincer movement of tablets and Chromebooks. That the two are advancing together may be either an accident or a deliberate attempt to establish distinct post-PC categories—all we know for sure that Google likes to experiment publicly.

The Galaxy Tab

The Galaxy Tab 10.1 is a close match for—some might say it mimics—that proven PC-skewering weapon the iPad 2. The tablet that I reviewed is a special edition, with Android logos on the back, that was handed out to developers and lent to journalists at the Google I/O conference last month. You can buy it without the decoration for $500 with 16 gigabytes of storage or $600 with 32 GB. It’s WiFi-only for now, but a version with a cellular data plan is due out soon.

The Galaxy Tab’s similarity to the iPad 2 highlights the fact that in the tablet world, hardware is scarcely relevant. A responsive, glossy, color-rich touch screen, eight-hour-plus battery life, and front and rear cameras are all table stakes by now. The Galaxy Tab 10.1 is actually slightly slimmer than the iPad 2 (by 0.2 millimeters) and lighter (by 35 grams), thanks to the plastic back it has, instead of an aluminum one. It’s also more widescreen, with a 16:10 aspect ratio.

Google’s post-PC vision—like Apple’s—is all in the software, but this is where the similarity ends. Jobs’s claims about the first iPad’s “magic” were dismissed by those who saw the device as nothing more than a “giant iPhone,” and the iPad 2 can still be accurately described that way. When you turn it on, you are greeted with a grid of every app you ever installed. Customization doesn’t go beyond the ability to group the icons into folders and move six to privileged spots on a dock at the base of the screen.

The Galaxy Tab’s Honeycomb 3.1, however, seems to be gunning to replace the desktop experience with something that looks to be suspiciously like another one, albeit without a mouse. You can clutter your five desktops with app shortcuts to your heart’s content. You can add “widgets” (cut-down, interactive versions of regular apps) to that clutter to do things like provide a permanent view of your e-mail inbox or music player. This latest release of Honeycomb allows you to resize your widgets, an option that makes it possible to create a desktop-PC feel by putting, for example, a calendar and an e-mail inbox side by side.

Honeycomb even comes with a very Windows-like system tray—a  place where running apps can be seen and notifications pop up—in the bottom right corner. But it all adds up to a less slick experience than an iPad—there’s much more to tinker with, and you invariably leave things untidy. The Galaxy Tab 10.1 requires a steeper learning curve than the iPad 2.

The Chromebook

The second part of Google’s post-PC vision requires is even trickier to master.

I used the “stable” version of Chrome OS that comes with Samsung’s Series 5 Chromebook on Google’s prototype the Cr-48 notebook, released last year, which has much the same hardware. The Samsung Series 5 will set you back $430 with 16 GB of storage and Wi-Fi only, or $500 for the same with 3G added (yes, it has less storage than you can get with the Galaxy Tab).

Learning how a Chromebook works is pleasant enough at first, as you adjust to a computer that takes just eight seconds to switch on from cold, and one second to wake from sleep (a state it can maintain for over a weekwhen starting with a full charge). The machine may be physically lightweight and have stripped-down functionality, but unlike some netbooks, it provides snappy access to even complex Web pages and handles full-screen Flash video just fine. Its settings menu is delightfully spare and really highlights the fun of junking a lot of stuff you always assumed had to be there in an OS.

But you soon hit the post-PC limitation of this vision: not being able to store files on your computer or do anything while offline. Users are encouraged to “install” Web apps from the Chrome Web store, but that essentially means adding a bookmark. File storage is intended to be via online services like Google Docs or Google’s beta cloud Music locker. (Google has said some of its services will work offline by later this year.)

Two recent additions to Chrome OS help, enabling you to view files that are on a USB drive and play music or video from a connected device, but both feel very primitive. When you can’t get Wi-Fi, or use 3G if your Chromebook has it, this vision of post-PC computing feels post-apocalyptic: everything digital you (digitally) own is gone, and your only chance of getting it back is to reinvent the Internet from scratch.

When you look at them together, it’s clear that each of Google’s two takes on a world beyond the PC demands considerably more of users than the simple, singular vision promoted by Apple. You’re expected to take a more active role in managing the complexity (Honeycomb) or the limitations (Chrome OS) of your device.

A deficiency the pair have in common is a lack of decent apps: the Chrome OS and Android tablet app stores are pitifully bare. Google claims that both are about to be saved by waves of innovative apps from third-party developers, but it’s an argument that feels persuasive only for tablets. Android phones had a few delinquent early years while their app ecosystem got started. But the Galaxy Tab’s groundwork of a richly featured if somewhat complex OS has been laid, and it just needs more app developers to come and build. The foundations of Chrome OS, however, are not so complete. Here Google is relying on developers to create powerful Web apps that work offline even before its own apps do so, or the OS feels like a finished product.

The two claws of Google’s pincer movement against traditional PCs may each offer more features - and complexity - than the iPad, but only one, Android Honeycomb, feels capable of doing as much damage as Jobs’ magical giant iPhone.