I have heard 1/8 cup to 1/4 cup per gallon (depending on how carbonated you want), but that is not backed up by my personal experience. I will backsweeten my first non-still batch of cider in about 3 weeks, and I'll let you know how that turns out. Dextrose is supposedly preferable because of the "fizzy" quality of the bubbles, but any fermentable sugar should work.
As far as non-fermentable sugars go, I would recommend using Xylitol, a natural alcohol sugar. (Stevia also works, though it has its own flavor, which I personally like but you may not want in your brew). Splenda is sucralose-- nasty stuff. A solution of sucralose will eat through an aluminum pan; not what I want in my brew. Go with Xylitol instead.