Diabetics have repeatedly reported that they see no fluctuation in insulin levels with the use of splenda or aspartame. As although it is similar to sugar, splenda is not very soluble in water so it's hard for it to hit your blood stream to trigger high blood GLUCOSE levels(its not like splenda dimer is broken anyway).
Aspartame however, is a dipeptide methyl ester.
In ester in aspartame is hydrolyzed and methanol is released(bad) because it is oxidized to formaldehyde(can make you go blind and is anti-sceptic(kills just about everything), and can further easily oxidize to formic acid, which is one of the most potent organic acids. Anyway the amount of aspartame used to sweeten most drinks is very small so the actual amount of methanol consumed is quite small, even smaller than say, a tomatoe. And some say in small quantities it may serve beneficial function.
keep in mind that aspartame is a protein that can be digested, that means that it actually does provide 4kcal/gram, but you use sooooo little of it as it multitude times sweeter than sucrose that is essentially meaningless calories.