Is the variation in carbonation of soft drinks the variable that affects the effectiveness of the mentos reaction?
It's my understanding that the two key variables here are carbonation, as you suggested, and viscosity.

People performing this sort of demonstration usually use drinks completely sweetened with artificial sweeteners like aspartame or Sucralose, because they're much less viscous than their full-sugar equivalents, and especially than the high-fructose corn syrup variants.

Common artificial sweeteners are much sweeter than sucrose and glucose [1], so they're added in much smaller quantities and so affect the properties of the liquid less. 

The viscosity affects the ability of the liquid to have the rush of bubbles from rapid nucleation required for the fountain effect, and the carbonation affects how much impetus there is behind it.

For an good comparison, you could try three variants of (popular cola product), including a newer one partially sweetened with steviol (look for the ones with green labels: Pepsi Next and Coca-Cola Life are two examples): the wholly artificially sweetened one should jet best, then the part-steviol, part-sugar one, then the full-sugar one.


"Sucralose is about 320 to 1,000 times sweeter than sucrose, three times as sweet as aspartame and twice as sweet as saccharin." -- Wikipedia (I'm not sure how that's measured, but the sucralose figure comes from the FDA approval document cited on the Wikipedia page.)