Sugar and dietary sugar are the same thing. Dietary means something you eat. Not "diet" sugar like aspartame. I think you mean sugar substitute or artificial sweetener. Or alternatively you could use wording like nutritive sugar (table sugar, etc) vs. non-nutritive sugar (artificial sweetener).
Salt DOES make you retain water, however the weight you "gain" from it is not fat. It is water, obviously. So the gain is only superficial and just mildly irritating to see on the scale. Water can make your weight fluctuate up or down by a few pounds every day, but it isn't fat.
Diet soda does not contain nutritive sugar. Most artificial sweeteners by scientific definition aren't even technically sugars at all. They are structurally similar enough that your taste buds interpret them as "sweet", but your body is not able to break them down for use as energy. There is absolutely no scientific evidence that artificial sugars are worse than nutritive sugars. If you'd like to search a huge database of peer reviewed scientific literature - PubMed home
. There IS evidence that artificial sugars do not help you feel full, but if you're drinking a diet coke, I don't think you're looking to be feel full, you're looking to quench your thirst.