Not sure which of the new TV set-top boxes is right for you? Well, if you're already a Netflix or Blockbuster subscriber, maybe it's not a question of which box you want, but whether you really need one. Here's how TV set-top boxes stack up to old-school DVD-by-mail services.

Delivery
Here's the most tempting reason to snap up a TV set-top box—instant gratification. With services like Apple TV, the Xbox Video Marketplace, Vudu, and Amazon Unbox via TiVo, you'll typically be able to start watching your movie within minutes—if not seconds—of hitting the "Rent Now" button, while Vudu tees up its movies instantly, provided you've got a decent broadband connection.

By-mail DVDs, as I'm sure you're aware, require a bit more patience. If you live near a distribution center, you can typically count on getting the next movie in your queue within a day after the service receives the last disc you sent back (or at least, that's been my experience with Netflix in the New York area). Those in rural areas may have to wait for days longer. And while Blockbuster offers in-store trade-ins, let's face it—you can't ready argue with "right now." Winner: Set-top box

Note: Netflix offers about 7,000 "watch instantly" titles, but you can only watch them on a PC—not on your TV.

Selection
Set-top boxes might have the edge in terms of delivery, but online services like Netflix and Blockbuster blow away their digital competitors in the selection department. Apple TV and the Xbox Video Marketplace, for example, each offer fewer than 500 movies for rent, while Vudu and Amazon Unbox fare better with several thousand each. Netflix, on the other hand, has a staggering 90,000-plus DVDs in its library—pretty much everything, in other words. Winner: Movies by mail

Video quality
While most of the set-top box services say their standard definition movies look just as good as DVD, the truth is that downloaded SD movies typically look softer and blockier than their DVD counterparts—indeed, over-the-Net HD movies may look only a little better than DVD quality. Why? Because video files are heavily compressed for faster downloads, and that means video quality suffers.

Of course, I've seen plenty of poorly mastered DVDs, as well, and if you're talking about a drama like, say, "The Hours," then a standard-def download will probably suit you just fine. But if you want to see "Transformers" in all its HD glory, you can't beat the physical HD DVD (or eventually, Blu-ray) version. Winner: Movies by mail

Sound quality
Full 5.1 Dolby Digital soundtracks are somewhat hit-and-miss when it comes to downloadable movies. For example, the HD version of "Transformers" on Apple TV has a 5.1 soundtrack, but not so for 2000's action-fest "X-Men." Meanwhile, most of the SD movies on set-top boxes are relegated to matrixed, two-channel Dolby Surround only.

On disc, however, if a given movie was released with a Dolby Digital and/or DTS surround soundtrack (and most movies made after 1994 were), you'll most like get it on DVD, Blu-ray and/or HD DVD. And if you're into new surround formats like Dolby Surround EX or DTS-HD Master Audio, disc is the only way to go. Winner: Movies by mail

Extra features
Set-top box movies may arrive instantly, but if you want extra features, you'll be pretty much out of luck. A few of the online services are gearing up for basic extras such as, say, commentary tracks, but for the most part, chapter stops are the most you'll get. Cinephiles who want all the deleted scenes, storyboards, and "making of" documentaries will be better off with discs. Winner: Movies by mail

Prices
Let's start with the boxes themselves: Apple TV goes for $229 (40GB) or $329 (160GB), while the 250GB Vudu and the 20GB Xbox 360 go for $299 and $350, respectively. You can get a non-HD TiVo for as little as $99, but you'll also owe $13 a month in service fees. Meanwhile, you can get a decent DVD player for $50 or less, while top-notch upscaling DVD decks go for between $100 and $150.

None of the set-top box services offer subscription plans yet, so you'll have to rent a la carte, and prices are somewhat high: typically $3 for catalog titles, $4 for new releases, and $5 and up for HD movies.

Netflix and Blockbuster, of course, have their various "X number of DVDs at a time" plans, ranging from one at a time for about $9 to about $16-$17 for three at a time. (Blockbuster also has pricier plans that allow for in-store trade-ins.)

For the sake of argument, stacking the $17 three-at-a-times plan against a la carte movie downloads…the $17 subscription equals about four or five movie downloads, depending on your mix of newer and older titles. Rabid by-mail movie watchers, on the other hand, could probably squeeze in, oh...about 20 movies, depending on the reliability of their mail carriers. Winner: Movies by mail

And the winner is...
As I wrote above, it's hard to argue with set-top boxes when it comes to the instant gratification factor, and hey, they're cool gadgets—I've got both Apple TV and the Xbox 360, and I get a kick out of renting movies on them. And will downloads eventually take over the DVDs-by-mail business? Probably, yeah.

That said, dollar-for-dollar and taking into account selection, overall sound and video quality, plus bonus features. movie-by-mail services—low-tech though they may be—still take the cake. Winner: Movies by mail