Carbon Dioxide Solubility in Water
Sodium carbonate ($\ce{NaCO3}$) is basic and gives solutions with a pH of around 10.  Sodium bicarbonate ($\ce{NaHCO3}$) creates solutions with a pH of approximately 7.3.  This is in fact what the human body uses to regulate pH.  You want to used the sodium bicarbonate since it will yield a nearly neutral solution.