Regular pectin needs regular sugar to activate the gelling process. Low/No Sugar is a different
product which allows the gelling to occur with "no" (meaning artificial sweeteners like Splenda) or "low" which means "less" sugar than the regular. Most liquid or powdered pectins have you use equal parts sugar to fruit to insure a good gel set.
If you're at all inclined to get more experimental, there's a product called Pomona's Universal Pectin that requires NO sugar (unless you want some) one way or the other, it's its own gelling agent. Some of the canning companies (Ball, Kerr, etc.) are now coming out with different "measure as you go" gelling agents as well.
It depends on the kind of fruit you use, how you want to cook it (You don't have to cook fruit nearly as long to achieve thickness if you're using packaged pectin) and how much sugar you are willing to use for taste, dietary or other considerations.
I actually don't like the "low-sugar" options because the finished product looks "dull" as opposed to gel-like. The flavor, however can be superior because if the fruit is good, it is less masked by the sugar. Decisions, decisions. Let me know if I can help otherwise.
