On the surface (by nutrient breakdown), they look pretty similar except that a sweet potato has massively more vitamin A. According to Dr. Mirkin:
7-ounce white potato with skin: 220
calories, 5g protein, 51g carbs, 20mg
calcium, 115mg phosphorus, 2.8mg iron,
16mg sodium, 844mg potassium, 4g
fiber, .22mg thiamin, .07mg
riboflavin, 3.3mg niacin, 16mg vitamin
C
7-ounce sweet potato: 208 calories,
3.5g protein, 49g carbs, 56mg calcium, 110mg phosphorus, 1mg iron, 20mg
sodium, 693mg potassium, 5g fiber,
4350 RE vitamin A, .14mg thiamin,
.13mg riboflavin, 1.2mg niacin, 49mg
vitamin C.
One difference not captured in that breakdown is that the glycemic index is quite different at 85 (high) for white potato and 54 (medium) for sweet potato.