When it comes to sugar substitutes, splenda and aspartame are the two big players in the game. Aspartame was discovered in 1965 as a by-product of an anti-ulcer drug. Splenda is a relative new comer which was discovered in 1976 and is made by adding maltodextrin or dextrose to "bulk" it up. Both splenda and aspartame are fine white powder and are available in packets varying in size from handheld packets to five pound bags, so you can get just what you need to sweeten your tea or bake your pies.
Splenda is used in about 4,000 different foods and drinks, and since it is nearly 600 times sweeter than sugar, a little bit goes a long way. Splenda has the added advantage of being stable when heated so it can be used in baking and still retain its sweetness. However, in recipes that it put on top of it does not created the same texture as sugar so it does not work as well in recipes like creme brulee.
Aspartame is used in about 6,000 different foods and drinks (from supplements to soft drinks) currently on the market. Aspartame has phenylalanine in it, so people who have a rare genetic disease called pheylketonuria must avoid it. In addition, aspartame does break lose its sweet flavor when it is heated so it is not recommended for use in baking. Also, in things like hot cocoa, there can be a bitter taste if it is left to set awhile.
Splenda has shown no adverse reactions in testing until the amount consumed was equal to 4,000 packets a day. Aspartame has been a bit more controversial because there have been questions about the validity of the research done in the past. However, more than 90 different countries have found aspartame to be safe for human consumption. As late as 2007, a safety evaluation concluded that aspartame is safe to use as a sweetener with no nutritious value.
While both splenda and aspartame sweeten the foods they are added to, there are some people who report a bitter taste left in their mouths after eating foods made with aspartame. Splenda has no reported aftertaste and can be used spoonful for spoonful just like table sugar. In most restaurants splenda (usually in little yellow packets) is readily available, while aspartame (most often in pink packets) not being around as much.
When deciding between splenda and aspartame, it its important to remember that studies conclude that both are safe when consumed within reasonable limits. However, for baking, splenda wins hands down. In prepared items you may need to taste the foods to decide if you are one of those who gets an unpleasant taste left in your mouth after eating aspartame items.
With the aspertame testing controversies resolved it now boils down to personal preference and the way you intend to use splenda or aspartame that can help you decide which one is right for you.