By Rob Pegoraro

A tiny $99 box can connect your TV to a wealth of Internet-delivered TV shows, movies, pictures, podcasts and music. But which $99 box should you get?

In one corner, there’s Apple’s $99 Apple TV, and in other we have Roku’s slightly larger $99.99 XD-S–and its slightly less capable $79.99 and $59.99 siblings.

But although these devices may resemble each other, down to setup routines in which the only hard part is typing your WiFi network’s password on an on-screen keyboard, they represent different ideals.

Think of the Apple TV as a projector and the Roku as an antenna. Apple’s box functions largely as an extension of its iTunes store and the iTunes libraries on any computers in your house. The Roku, meanwhile, has evolved from its 2008 origins as a Netflix-only player to tune into a growing variety of online content sources.

To put this another way, the Apple TV is a fantastic device if you watch only shows on Fox and ABC–the only two U.S. networks to sign up for the 99-cent rentals Apple introduced with this device last month.

Browsing and searching through its listings is easy, although its remote’s tightly spaced buttons make it too easy to select something when you meant to navigate. Over a Fios connection, shows appeared in seconds–free of commercials and playing in high definition that actually looked it, unlike the blurry “HD” that airs on many Internet video services.

(If you have a slower connection, Apple says the Apple TV can cache a show or a movie on its hard drive in its flash memory until you can watch it uninterrupted.)

Renting movies, starting at $2.99 for standard-definition titles and $3.99 for high-def fare, is just as easy. But Apple’s selection of rentals, like those of every other video-on-demand site, suffers from the limited availability imposed by Hollywood’s idiotic “release window” business model. Want to rent “The Hurt Locker” or “The Hangover”? Sorry, too late.

Both TV and movie rentals give you 30 days to start watching; you have 24 hours to finish watching a movie and 48 hours with a TV show.

You can also watch purchased iTunes TV shows and movies–and play back music and view photos–through a copy of iTunes on another computer at home. But you have to remember that the Apple TV doesn’t work off the “Sharing” option listed in iTunes’ preferences; instead, you need to enable the separate “Home Sharing” option hiding under the Advanced menu.

In addition, you can watch Netflix TV shows and movies, play short clips off YouTube and view photos from Flickr. And that’s about it–there’s a Web-radio function, but its lack of a search feature makes it useless. Even if you’ve bookmarked Web-radio stations in iTunes, you can’t play them through the Apple TV unless you set up Apple’s Remote application on an iPhone, iPod touch or iPad (which also gets around the limits of the Apple TV’s own, cramped remote).

The next version of the iOS software in those devices will bring a fascinating feature called AirPlay, which will let you watch video playing in many iPhone or iPad apps on your HDTV. That could make the Apple TV much more interesting. So could updates to the Apple TV’s own software … assuming Apple doesn’t neglect this model as it did the original, now-abandoned Apple TV.

Roku’s boxes don’t have the Apple TV’s constraints or all of its elegance. This Saratoga, Calif., company seems to have put more effort into signing up content partners for its Channel Store than to optimizing the experience of browsing that catalogue.

The selection available on Roku’s XD-S, XD and HD–plus older versions of its video players–is certainly impressive. Among its 87 channels, you have such name-brand entries as Netflix, Amazon’s excellent video-on-demand site (with pricing that often beats Apple’s), the Pandora Web-radio service, Flickr and Major League Baseball’s MLB.tv–plus, coming later this fall, Hulu’s $9.99/month Hulu Plus catalogue of TV shows, a cheaper way to catch up on TV than iTunes rentals.

You can also browse through dozens of quirkier offerings: NASA TV, tech podcasts, church sermons, news broadcasts and more. YouTube, however, goes mysteriously missing; the artsy video-sharing site Vimeo can’t really fill its spot.

Even harder to explain: The lack of a search function on either Roku’s TV-screen interface or its site, which limits you to browsing through the selection, one channel at a time.

Another puzzler comes from the Roku player’s poor connectivity to the media libraries on your own computers. Even though one of Roku’s first devices was a wireless music receiver that connected automatically to your shared iTunes libraries, it doesn’t provide an equivalent function on its current lineup, leaving users to experiment with third-party solutions listed in the Channel Store.

The XD-S does include a USB port, but this, too, gets poor support by Roku’s software. I could barely believe the tech-support note that counseled users to go to the company’s site to activate a “private” channel that enables this hardware.

After I did that, I needed to restart the player before it would recognize any of the flash drives I tested. Photos and music played back without any problems, but video had issues–a copy of “The Matrix,” ripped from a DVD using the free HandBrake program, didn’t feature more than brief snippets of sound.

The other major advertised step-up feature for the XD-S, playback of 1080p high-definition video, suffers from a severe lack of 1080p content online. So most people will be fine with the $59.99 Roku HD, which connects to the same sites as the XD-S and in a quick test felt like the same device.

Both the Apple TV and the Roku can not only help trim or eliminate a cable or satellite subscription, they can also lower your electric bill. The XD-S drew 6 watts in use and the Apple TV only used a single watt–just a fraction of the juice a cable and satellite receiver can suck down (PDF).

Have you bought either of these gadgets? What do you think of them so far? What’s on your wish list for the next software update?