Go light on the butter and sour cream, and sweet potato is definitely better than starchy white potato. However, in my experience, if you back up those carbs with a good serving of lean protien AND a medium salad or bowl of broccoli or cauliflower-- no corn or carrots as these veggies are packed with natural sugars!---I say feast and enjoy without remorse!
It depends. potatoes are a simple carb that will raise your blood sugar, but plain potatoes with no butter or milk are still very low in calories. As long as it is a sensible diet with a good veg/protein profile, potatoes can be a good part of it. Sweet potato would be more nutritious though.