For many of us who travel and want to get online from the road, Wi-Fi hotspots alone just don't cut it. There aren't enough of them, they aren't always available where you want them, and they can be a security nightmare. There is a better alternative: cellular data service from the same wireless carriers who connect our cell phones.

The advantage of cellular data service is its ubiquity. Like cellular voice service, it's based on networks of base stations that are located in most cities and airports and along most major highways in the United States. That means you can get online at near-broadband speeds in most metropolitan areas, and at slow but still usable rates in less-traveled and less-populated places.

Until recently, the biggest problem with cellular data service was that few of the carriers offered good Mac support; Verizon was long the main exception. But that has gradually been changing: all the major carriers now make it much easier for Macs to get online, too.

This means that for most mobile Mac users, a cellular data plan is now the best--if not the least expensive--way to get online from the road. But which carrier's service should you choose? Here's how they compare.

The Carriers

AT&T, Sprint, and Verizon each offer speedy, third-generation cellular data networks in the United States (see our cell data glossary). Sprint and Verizon claim downstream rates from 600 Kbps to 1.4 Mbps; AT&T, from 700 Kbps to 1.7 Mbps; all three can reach peak speeds of more than 2 Mbps. Going the other way, Verizon and Sprint claim upstream rates of 500 to 800 Kbps; AT&T, from 500 Kbps to 1.2 Mbps.

Those claims aren't too far wrong. In informal testing, our colleagues at Computerworld found that downloads averaged around 500 to 750 Kbps, peaking around 1.2 to 1.6 Mbps, while uploads were about 230 to 480 Kbps. In my own informal testing with a new Sprint adapter card, I actually hit a peak of 2.4 Mbps for one download.

All three services provide roughly comparable coverage. Of the three, AT&T is lagging furthest behind: It has full service in 275 of the 350 top metropolitan markets, and plans to finish rolling out service in the remaining 75 this year. When you roam beyond the reach of the 3G network, service drops back to whatever last-generation cellular data network that carrier used; as a result, your speed will drop more precipitously on Verizon's and Sprint's networks than on AT&T's.

T-Mobile is lagging behind the other big national carriers: While the company just launched a 3G network in New York City, it has only an intermediate 2.5G network--EDGE--in the United States. (EDGE is the middle-speed network used by the first-generation iPhone.) T-Mobile is also alone among the Big Four in not offering cell-data options for Mac owners; it offers just a single PC Card, with no Mac drivers or software.

The Plans

The cost of 3G service depends on your carrier's

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