Some RNYers 'dump' on sugars---this is an insulin reaction that's very unpleasant. Some of them also react badly to fats as well, but this isn't true dumping---it's probably bile-related. But MOST people with the RNY avoid sugars and fat because of CALORIES, pure and simple.
As a DSer, I eat full-fat over low- or non-fat every time. When a food item has fat removed, that fat has to be replaced with something---and it's usually carbs. As a DSer, I'm only going to absorb about 20% of the fat I eat (and if I don't eat enough of it, I get constipated!), so why would I choose a higher-carb, more expensive, and (for me) less healthy alternative?
I don't do aspartame if I can avoid it at all---I dislike the taste of it, and it gives me headaches. I've NEVER done 'diet' sodas---if I'm going to drink soda, it's going to the 'The Real Thing' (yes, Coca-cola Classic!), not that I often drink soda. (I just don't like it like I did pre-op.)
I drink tea sweetened with Splenda all day long. I avoid sugar alcohols, too---bad gas! I eat things sweetened with real sugar, but in moderation. I make home-made ice cream with Splenda and heavy cream---you can even add protein powder to the recipe!