Kuan is right. Isomalt is one of many sugar alcohols. Others you might know are Erythritol, Sorbitol, Mannitol, Maltitol, Xylitol and Lactitol. In a formula they could be used in amounts similar to sugar. But they are very expensive compared to sugar, $1 to $3 a pound. Also, they have a funny side benefit called the "laxative effect." If you ever eat candies with Mannitol I suggest you stay very near a toilet. The least laxative effect is Maltitol. The sweetening effect for sugar alcohols ranges from about 30% to 90% that of sucrose (table sugar) depending on the type.
Intense sweetners are another animal altogether. Sucralose (Splenda) is one. Aspartame (Equal) is another. Then there's Acesulfame K (Sunette) and Saccharin (Sweet 'n Low). Also Cyclamate, which was banned in the US but will be making a comeback.
Intense sweetners' sweetening effect is anywhere from 200 to 800 times that of sucose (on a equal weight basis.) Others coming on the market go as high as 8000 times. When you use Splenda or Equal in a small packet you are not using pure intense sweetner. It is always cut with dextrose or maltodextrins. The amount of intense sweetner in the packet you could fit on the tip of a pin. Pure Sucralose costs about $150 a pound.
Lately I see some large bags of some "Splenda" material in the supermarket for home baking. This is nothing more than very fluffy maltodextrin with a minute amount of intense sweetner going for a very high price. I wouldn't waste my time with it.