The Fast Forward column in the Aug. 28 Sunday Business section listed two figures for the number of songs available at the Napster music-download store. Only the first number, more than 1.5 million, should have been cited. (Published 9/1/2005)

Picking an online music store ought to be no more complicated than choosing between grocery stores. But if you're going to buy more than a few songs a month, you may find yourself in the kind of long-term commitment associated with inking mortgage documents.

That's because the big online music stores (for example, Apple's iTunes Music Store, Microsoft's MSN Music, Napster, RealNetworks' RealPlayer Music Store and Rhapsody, Sony's Connect, Yahoo Music and Walmart.com Music Downloads) don't all sell the same bundles of bits.

Instead, they offer songs and records in different file formats. Almost all of these restrict you to certain programs and music players, and almost all arrive cocooned in a layer of copy-control software to stop you from sharing your purchase with the neighbors.

You can't quickly or conveniently convert these files to another format. Your downloads today may dictate the hardware and software you buy years from now -- and conversely, what you own now can determine where you download in the future.

This applies to anybody with a Mac or an iPod: The iTunes Music Store is the only major online shop to offer full support for Apple's hardware. But if you've bought one of Sony's digital-music players, the only store with compatible downloads is Connect. And if you own a player compatible with Microsoft's Windows Media Audio files, you'll do best at sites that sell music in the same format.

Those factors aside, here's what to look at when considering these stores.

Inventory: The selection at the weakest online store dwarfs that of the biggest land-based store. But we've all been spoiled by the likes of Amazon.com; we rightly expect an online store to carry every song ever committed to vinyl, tape or CD.

You can blame the music industry for some of these gaps. Some artists only let their work be offered for download at one store, while others don't allow their music to be sold as bits at all -- an act of denial that would make the Flat Earth Society proud.

But stores share responsibility too; not all have signed deals with enough record labels or put full effort into stocking back-catalogue material.

Apple and Napster both claim inventories of more than 1.5 million songs. Real's stores stock a bit more than 1.2 million; MSN Music, Napster and Yahoo cite catalogues of more than 1 million songs; and Wal-Mart carries about 600,000 songs.

But none of those numbers means anything if a store doesn't carry the one artist, album or song you want. Searching through the catalogues of Apple's store requires downloading and installing its own software; other stores let you window-shop.

Pricing: All of these stores sell songs for 99 cents each (except Wal-Mart, which charges 88 cents), with per-album rates typically ranging from $9.99 to $13.99 or more. In other words, a single should cost the same everywhere, but you can pay more for an album at one site than at another. But there's no clear pattern to this; temporary promotions excepted, nobody has a real edge.

Usage rules: Most major sites let you copy a downloaded song to as many music players as you want and play it on varying numbers of computers at any one time -- seven for iTunes and MSN, five for Napster, RealPlayer and Yahoo, three for Rhapsody and Wal-Mart. You can also burn it to CD as often as you want, except that any given playlist may only be burned to disc five to 10 times.

Real and Rhapsody fall a little short, however, as their CD-burning limits apply to any given set of songs, regardless of order. Sony offers an extra outlet: You can fill a data CD with the original ATRAC-format files bought at Connect -- fitting hundreds of songs on the disc in the process -- then listen to it on some of Sony's CD players.

Software: All these stores require using a program to download and sort songs, copy them to portable players and burn them to audio CDs. ITunes remains the best of this bunch, trailed by Yahoo's Music Engine and Microsoft's Windows Media Player (used in stores from Wal-Mart to MSN). Napster's software is geared too much toward shopping, Real's teems with ads and Sony's SonicStage is just horrid.

Playback on your stereo: Once you've piled up a nice little stash of downloads, burning them all to audio CDs just to hear them through the good speakers in the living room will get inconvenient. A "media receiver" -- a compact box that plugs into a stereo system and connects to your wired or wireless home network -- can play these files off your computer's hard drive.

Windows Media-based stores such as MSN, Napster, Rhapsody, Yahoo and Walmart.com have a huge advantage, thanks to the wide variety of receivers that support this format. With iTunes, your only option is Apple's AirPort Express, which lacks a remote control or a display.

Sony doesn't offer any such capability.

Rent or own? Napster, Rhapsody and Yahoo allow you to pay a monthly fee to download unlimited "tethered" copies of almost every song in their catalogues. These files can't be burned to CD but play on computers and some newer Windows Media-compatible players.

Napster's $14.95-a-month Napster To Go offering, the first of this kind, has been outclassed by Real's Rhapsody Unlimited, $8.33 to $9.99 a month, and particularly Yahoo's Music Unlimited. It costs just $4.99 a month for a one-year commitment, with a generous discount on song purchases.

These subscription plans are great for auditioning new music or for building temporary collections (say, a soundtrack for a party). They're bad for keeping songs around, as you must keep paying to preserve access to songs you've downloaded. (I've already heard from one reader who's been locked out of his rented songs by a server glitch.)

All that in mind, iTunes -- the first store to make downloaded music a commercial reality -- still has the lead. It offers the best selection, permissions and software, plus such extras as the ability to print CD covers and track lists, downloadable liner notes and videos for some albums and a directory of free podcasts. And it works with the iPod, the best music player out there.

The only knock against iTunes goes back to one of Apple's least appealing traits: its tight-fisted control over its products. There could be some great iTunes-compatible media receivers, car stereos or home theaters out there, but we'll never know until Apple invites other companies to develop products that play iTunes downloads.

If Apple's exclusivity turns you off, take a look at MSN Music. It's the next best thing, thanks to its good-as-iTunes use permissions and clean, Web-based interface (even if that interface requires you to use the aging Internet Explorer browser to buy anything).

If you want to try a subscription-based service, go with Yahoo. Its prices make Napster and Rhapsody look silly, and it offers a remarkably pleasant music-playback program in its Yahoo Music Engine.

If contemplating all these compatibility issues bothers you, and if you've got a taste for more obscure music, consider the smaller stores that sell unrestricted, universally compatible MP3 files. Sites such as eMusic.com, Smithsonian Global Sound, Calabash Music and Download Punk can do this because they don't carry music from the major record labels that often act as if they regard customers first as a piracy risk to manage, second as a source of profit.

Or get yourself in the habit of burning new purchases from the major online stores to audio CD, then copying them back to your computer as plain old MP3s.

It seems too much to hope that we'll ever get back to having every download play on every computer and player -- just as CDs work everywhere. But stranger things have happened: Only a few years ago, the very existence of a dollar-a-song store that carried 1.5 million tracks seemed implausible.

Living with technology, or trying to? E-mail Rob Pegoraro at rob@twp.com.