Here's the deal. Cool Whip has sugar, but heavy cream has fat. The fat in Cool Whip is Trans fat, and therefore the worst kind. There is no perfect SBD safe substitute for whipped cream, but if you get the Cool Whip Free, which has, in a single serving, no fat, you are getting no fat and very little sugar. You could also make your own whipped cream from heavy cream and splenda, but then you have a great deal of fat. It's a toss up. Personally, even small bits of sugar can affect me greatly, but the Cool Whip Free never has, since I do keep it to a small amount.
Hope that helps...it's definitely one of the confusing things about SBD!