A real story here:
My mom, who did little to no exercise, had a 12 pack of coke a day habit, and was a little overweight(typical 60 y/o female in America). After the doc said her triglycerides and cholesterol were too high, he suggested she switch to diet coke(otherwise her diet was OK).  Bam, 6 mos later, she's 20lbs lighter and blood work is within normal range.  Diet coke did not make her fatter!  She lost weight, just by changing that 1 thing in her diet.  I'm not suggesting anyone drink 12 everyday, but if you have the choice, a 0 cal beverage would be better.