My husband being a diabetic, generally avoided all dessert items at WDW - - unless it was something he had planned into his diet. For the most part, he preferred to have just a forkful of something really good (if it was a buffet) and be careful of other foods he ate. I found that a lot of the no-sugar added desserts still had plenty of carbs...after all, no sugar added apple pie or even a pudding, still has carbs and can spike him up - he would rather just have quality over quantity.
The one dessert that I had was at Grand Floridian Cafe and it was a no-suggar added fresh berry/gelatin (but really really top notch quality) triangle. I know that I tasted artifical sweetener in it - - I would guess (just a guess on my part of eating so many different ones) aspertame. This was a good choice for diabetics who can handle artifical sweetner and can count the fruit carbs (raspberry and blueberry).
Good luck!