The difference between aspartame and sucralose is the chemical composition of each compound. Aspartame is made from two amino acids, while sucralose is made by modifying the chemical makeup of sucrose or sugar.
Aspartame is made by combining the two amino acids aspartic acid and phenylalanine. The combination of these two amino acids creates a new compound that is very sweet, but without calories. The amino acids are broken down in the body and absorbed like amino acids in protein. Aspartame is about 200 times sweeter than regular sugar.
Aspartame breaks down quickly at high temperatures and is not suitable for baking or cooking because of this. Aspartame can also sometimes leave a bitter aftertaste, especially when used to sweeten hot things. Nutrasweet is the brand name of aspartame.
Sucralose is made by replacing the hydrogen and oxygen pairs in sucrose with chlorine molecules. This change makes sucralose much sweeter than sugar and eliminates all the calories. Sucralose is about 600 times sweeter than sugar, so a very small amount is needed to sweeten foods. Sucralose is stable at higher temperatures and can be used for baking and cooking. Sucralose is sold under the brand name of Splenda.
Both aspartame and sucralose are used as artificial sweeteners in place of sugar and both have zero calories. Many people who are trying to manage their weight use these sweeteners in place of sugar to limit their caloric intake.