Smart phones, which combine a cell phone with a PDA, are moving out of business class into the coach cabin.

Long used by corporate travelers to keep up with e-mail and appointments, smart phones are now catching on with consumers. Some want to tap out text messages on a QWERTY-style keyboard rather than the tiny keypad of a cell phone. Others see no reason to tote both a PDA and a cell phone when a smart phone can do the job of both.

Catering to this growing interest are smart phones that are slimmer and sleeker, with simplified interfaces and setup menus geared toward users who like things fast and easy.

Decide how “smart” a phone you want. We’ve grouped smart phones into two IQ categories. What we call advanced smart phones offer laptop-like capabilities in a palmtop-sized package. They let you create and edit spreadsheets and text documents, and they usually come with Microsoft Outlook, Palm Desktop, or other personal information management (PIM) software for your PC. They typically have touch screens for accessing the phone’s many features. They also let you load data onto the phone using standard-sized SD (Secure Digital) cards rather than “micro” versions found on basic models.

Basic smart phones resemble regular phones, and have fewer features than advanced models (they typically lack touch screens and PIM software, for example, and don’t allow you to create or edit documents and spreadsheets). They’re fine for reading e-mail, but composing and sending them is more cumbersome.

Consider the network. CDMA-based advanced smart phones are best if you frequently download data-heavy attachments, because such models use the high-speed EV-DO network from Sprint Nextel or Verizon. The slower, GSM-based EDGE network used by T-Mobile and Cingular is best suited for e-mail with small attachments or light-duty Web surfing, though Cingular has begun deploying a higher-speed data network called HSDPA.

Consider the operating system. Smart phones run on various operating systems, each with its own character.

Size up the keypad and display. A smart phone’s shape and size are largely determined by its keypad and display. Some models have a full keypad that slides out from behind the phone and tucks away when not in use. Others--basic models such as the BlackBerry Pearl--leave the keypad in plain sight, but keys do double and even triple duty. Still others, such as the Nokia 9300, open like an eyeglass case to reveal a full QWERTY keyboard. Which type is more convenient depends on the functions you use most.

Consider the plan. Using the extra, network-dependent capabilities of a smart phone requires both a regular (voice/text) phone plan and a data plan for Web surfing and sending and receiving e-mail. Depending on the carrier, prices for both combined start at $45 to $80 a month with a two-year contract. But you can easily spend more than $200 a month as you add minutes, text messages, and other services.

If you already have a “family” plan, you may have to move your smart phone’s number to a new account, as we had to with Verizon and T-Mobile.

Price and flexibility make Sprint’s smart-phone plans the best overall. T-Mobile’s are also relatively inexpensive. Cingular’s cheapest plans have a megabyte limit, above which you pay a higher rate. Verizon’s plans are the most expensive and the least flexible.