I saved Carley because she saved my ass previously when that zombie jumped Clem in the drug store. Wishful thinking had me hoping her gun could save Doug too.
Also, she was real cool about keeping the whole 'convicted murderer' thing on the down-low.
Either way it was a damn tough call to make as they both seem to be super friendly, super useful characters to have around. Personally I really enjoy those difficult split-second decisions with no obvious right or wrong answer.