I usually fish out whatever card is giving me the best benefits. Right now it's my Target Visa (for every $2500 spent I get a 10% unlimited day off at Target. I easily turn that into a $200 return

) and my BP card - 10% off BP gas, 5% off food and travel (just booked 3 plane tickets) and 2% off everything else. That's good until April, I believe - need to check the next statement.
Spiritair.com also recently started serving ATL so I signed up for their card which gives 15K miles for the first purchase and 3X mileage on tickets bought on their airline. With first class tickets from ATL to LAS for about $350, they might be a good switch up to Delta. They have flights to some interesting places I haven't been to yet
I put all my business stuff on personal cards since I'm an S-corp. Never really saw a need to get a business card.