I sometimes buy yams when sweet potatoes aren't available and wondered the difference in nutrition. Here is what I found to my surprise. The Vitamin A difference is huge.
YAMS 
Serving Size 1 cup, cubes (150g)
Amount per serving
Calories 177 Calories from Fat 2
Hide Daily Values % Daily Value* Total Fat 0g
0% Saturated Fat 0g
0% Cholesterol 0mg
0% Sodium 14mg
1% Total Carbohydrates 42g
14% Dietary Fiber 6g Sugars 1g
Protein 2g
Vitamin A 4% Vitamin C 43% Calcium 3% Iron 5% Thiamin 11% Riboflavin 3% Niacin 4% Pantothenic Acid 5% Vitamin B6 22% Potassium 35% Phosphorus 8% Magnesium 8% Zinc 2% Copper 13% * Percent Daily Values are based on a 2,000 calorie diet.
SWEET POTATOES
Serving Size 1 cup, cubes (133g) 1 sweetpotato, 5" long (130g)
Amount per serving
Calories 114 Calories from Fat 1
Hide Daily Values % Daily Value* Total Fat 0g
0% Saturated Fat 0g
0% Cholesterol 0mg
0% Sodium 73mg
3% Total Carbohydrates 27g
9% Dietary Fiber 4g Sugars 6g
Protein 2g
Vitamin A 377% Vitamin C 5% Calcium 4% Iron 5% Thiamin 7% Riboflavin 5% Niacin 4% Pantothenic Acid 11% Vitamin B6 14% Potassium 13% Phosphorus 6% Magnesium 8% Zinc 3% Copper 10% * Percent Daily Values are based on a 2,000 calorie diet.