There are basically three kinds of cards that earn award travel.

One kind is linked to one airline or hotel program. Most of these earn one airline mile per $ spent (hotel point credits vary) and charge an annual fee. A few exceptions, no fee but a lower earning rate, were mentioned in prior posts. If you charge a lot, a fee is worth it. An advantage of these cards is that miles/points can be used for awards that can be prohibitively expenive to pay for, such as overseas travel in business or first class. Most of these are MC/Visa, with a few AmEx Optimas (e.g., Delta's card).

Another kind earns credits that can be converted directly to airline/hotel points. These include some AmEx and Diner's cards. Starwood AmEx is the no-fee leader here. It offers a 25% mileage bonus if you convert 20K points to the same program at the same time. Biggest drawback is low acceptability versus MC/Visa. Most people who have one of these get an MC/Visa as well. Before you get one, make sure its convertibility includes your desired airline(s).

The third kind accumulates credits in its own program which must be redeemed directly through their own agency. Biggest drawback is that awards are effectively limited to inexpensive economy tickets. If your travel qualifies for these, you can get tickets for fewer charged $ with one of these cards than with an airline program. (And these tickets, since the credit card company pays the airline for them, usually earn miles.) If you want any other kind of award, forget them. Upgrades and such are either unavailable or absurdly expensive, and you can't combine miles earned through flights or hotel stays with "miles" earned for charging on the card.

So, first you have to decide which general kind of card you want. Then the pros and cons of specific cards within the category matter.