The FDA has approved the use of sugar substitutes, such as aspartame. Most individuals can use all FDA-approved sweeteners, but saccharin is not suggested for pregnant women because it can cross the placental wall. Salt modification may be used to help treat those with hypertension, but continuos high salt intake generally does not increase the risk of developing hypertension. Unfortunately, the answer is very subjective. The negative factors of both butter and margarine have to be considered.
Butter contains saturated fatty acids that can elevate blood cholesterol.
Margarine contains trans fatty acids, which can also have an adverse effect. The decision's up to you.