For now, Diet Pepsi remains the No. 7 carbonated soft drink with 4.9% of the market, according to Beverage Digest. That's down from 5.3% in 2000. Meanwhile, Diet Coke's share has increased in that time from 8.7% to 9.6%. Diet Coke, which still only uses aspartame, overtook regular Pepsi to become the No. 2 soda brand in 2010.