Airline miles are great if you're looking to earn rewards that you can redeem towards award flights on your favorite airlines. They can also typically be used for seat upgrades, lounge access, and much more.
Most major airlines offer co-branded credit cards that allows you to earn a certain amount of miles per $1 spent. These cards can also grant additional benefits with the airline such as a free checked bag and priority boarding.
If you're looking to maximize your earnings, several travel rewards programs allow you to transfer your earned points directly to frequent flyer programs. In many cases this can earn you airline miles faster than using a co-branded airline credit card.
If you're loyal to a specific airline or two, I recommend considering an airline credit card. If you're not loyal to any given airline, you'll more than likely be better off with a travel rewards card.