I would probably take the cranberry juice with the teaspoon of sugar. But I'd probably use sucralose or saccharin and end up in a whole different discussion. Plus I probably wouldn't be drinking juice in the first place.

I don't think the fiber really does much, or at least not much in the "fiber as an antidote to sugar," sense, I think I've seen Lustig suggest something along those lines. If you look at the glycemic index of oranges vs orange juice, it's 40 vs 50, apples vs apple juice it's 39 vs 44. I think the structure might have an effect, you can pour juice down your throat and barely taste it, chewing forces you to experience the food. So maybe it is the fiber, but it's the structure it provides, rather than the slight difference the fiber makes to absorption.