I checked the Stonyfield website, looks like ounce for ounce, the greek wins.
Greek plain fatfree yogurt = 130 calories/cup, only 95 sodium, 350 potassium, only 9 carbs, 9 sugar, and a whopping 23 grams protein. 30% daily recommended calcium, riboflavin, and phosphorus.
Stonyfield's normal plain fatfree yogurt = 120 calories, over 100 sodium, 400 potassium, 11 carbs, 11 sugar, and only 9 grams protein. Also only 25% riboflavin and phosphorus.
It's all about the bang for the buck. They're both fat free, and the caloric content is similar enough that the difference doesn't really matter. But the greek has enough protein to cover 1/4 of your entire daily needs in a single 8-ounce serving. Plus - the greek's serving is 8 ounces, not 6, so it'll fill you up more. I adjusted the 6-ounce plain upward to an 8-ounce serving, so that the comparison would be between equal measurements of product.