Our questions don't get any simpler than this: "What is the best airline mileage credit card to apply for?" Unfortunately, while the question is simple, the answer is not. "Best" means many different things to different travelers, and one size definitely does not fit all. For that reason, I've developed a chart to help each of you answer that question for yourselves. I cover the three basic types of cards that earn "free" air travel:

• Airline cards are MasterCard and Visa cards issued by a handful of large banks, each co-branded with one individual airline. These cards are available for five legacy lines (American, Continental, Northwest, United, and US Airways) plus AirTran, Alaska, America West, Frontier, Hawaiian, Midwest, Southwest, and a bunch of foreign lines. AmEx issues similar cards for Delta and JetBlue. You receive credit (usually one mile per dollar) for every purchase you charge to the card plus bonuses for air tickets and other specified purchases; JetBlue and Southwest provide roughly equivalent awards. The credit you earn through the card goes directly into your frequent flyer account with the co-branding airline and mixes with the miles you earn by flying.
• Bank-buys cards are MasterCard and Visa cards, issued by various banks, and not affiliated with any individual airline. Instead, you accrue points based on how much you charge in a separate frequent flyer account with the bank; you cannot combine these miles with the miles you earn by flying in any airline's program. When you accrue enough points, the bank buys you a ticket on any of several airlines, at the lowest available price, usually with a dollar maximum for each ticket class.
• T&E (Travel and Entertainment) cards from American Express and Diners Club were the original travel-oriented credit cards. Both award one point per dollar (with some promotional bonuses), which you can convert into miles in several airline programs.

In the chart below, I've outlined the strengths and weaknesses of airline cards, bank-buys cards, and T&E cards. Each type of card is rated based on several factors, including how you use credit, how you earn credit, financial factors, and a few other factors. Take a look, and then I'll explain my evaluations below.

(1) Poor on Diners Club (2) Fair on most AmEx

Here are some of the details behind the chart's entries:

How you use credit

If your main interest is economy-class travel, the miles required for a bank-buys ticket are about the same as the airline miles required for a restricted frequent flyer seat, but your chances of getting a seat are much higher with the bank miles. The airline cards are rated "poor" because (1) travelers find it so hard to get restricted frequent flyer seats when and where they want them and (2) to bypass those limits, travelers have to use double miles.

On the other hand, the only practical way to the front cabin—through upgrades or premium-class awards—is to use airline miles. The bank-buys cards are virtually worthless for premium-class flying.

If you like to build up your credit over a period of many years, the T&E cards are your best option, since their credit never expires (as long as you keep your account in good standing). Also, while most airline credit expires if your account is idle after three or fewer years, your accumulated miles remain good as long as you keep using your card. But most bank-buys credit expires after a few years.