This is a subject that almost can be turned into science due to all the offers and variations. The theory is that if you travel with the same airline all the time or at least the vast majority of flights, a reward program specific to the airline do make sense.
However if, like in my case, I fly with the airline that offers newest aircrafts and second matches my travel times best it may be better with a airmile credit card that can be used on various airlines. After all, if you get 2 miles for every dollar spent on a CC, it adds up to 52 800 miles with spendings of 2 200$/month. Be aware of the downside of this solution if there are interest adding up on your monthly spending. However, if you can control finances and only pay the annual fee, this is a great solution.
Some have actually made the airmile credit card airmiles into a science, and may add even more benefits and more solutions to a cheap (or even free) flight. Flyfreeacademy.com have cracked many of the codes to make a system for those who both love to travel and would like to know how to balance the financial side of the reward programs.
Derek have spent a lot of time figuring out how airmiles ca rewards work. My experience will become guesswork compared to his solution so please check it out.
However the Venture card from Capital One is one of the most popular cards regarding airmiles and will be found on top-3 on almost all websites scoring CC vs. miles. I do belive that American Airlines have done a really good job with the upgrade of their aircraft (certainly better to fly AA Heathrow-Dallas than old A330 Finnair Chicago-Helsinki) both short and long range, also a big plus on professional staff. However they also have one of the highly recommended systems to gain free miles and you should check this out if AA have a schedule from your closest aiport. They also work with the One-world alliance wich do give even more benefits both on destinations and other solutions/services.
No matter the benefits, if you like to travel, checking airmiles that comes as a bonus on spending you would have to do anyways will feel really good!!!
Enjoy your miles in the air!