Updated July 17, 2015.
Yogurt has been touted as a health food for a number of years, and that might be true because it's an excellent source of calcium and protein. It also contains beneficial probiotics, which are friendly bacteria that are able to take up residence in your digestive tract (which is a good thing).
Frozen yogurt is a sweet concoction made of milk, yogurt cultures, sugar and flavorings that can be either artificial or natural.
It's similar to ice cream in texture but is usually lower in fat, so frozen yogurt became popular when low-fat diets first became trendy.
Being low in fat, especially saturated fat, is a good thing, but frozen yogurt is also high in sugar, so it still has lots of calories, and I'm not sure how much probiotic is found in any particular brand. If the probiotics are heat-treated any time during the manufacturing process, then they're probably mostly dead and dead bacteria don't do much good. You can look on the product label - if you see something about containing live cultures, then you know there is at least some probiotic bacteria in the frozen yogurt.
Nutrition Comparison
The nutrition information for yogurt varies a bit based on fat content and flavorings. One-half cup of plain yogurt made with whole milk has approximately 75 calories, 4 grams protein, 6 grams fat (2.6 gram of which are the saturated kind) and just under 6 grams carbohydrates, mostly in the form of milk sugar or lactose.
It also has 148 milligrams calcium, and 15 milligrams magnesium.
One-half cup flavored frozen yogurt has 110 calories, 2.6 grams protein, 3 grams fat (2 of those grams are saturated fat), 19 grams carbohydrates (much of which come from added sugar), 87 milligrams calcium and 9 milligrams magnesium. So, frozen yogurt has more calories from sugar and less calcium and magnesium than the same amount of plain yogurt.
And the Winner is...
So the plain yogurt comes out as the nutrition winner, but if you have a craving for sweets, the tangy flavor of regular yogurt isn't going to cut it. You can add fresh fruit or berries, though, or just a few extra calories and lots of vitamins and phytochemicals. If it's still not sweet enough, you can add a little honey - but not too much, or you'll start to pile on the extra calories from the natural sugars found in the honey. You can also add some sweetness with a little bit of zero-calorie sweetener such as sucralose, stevia or aspartame if you like.
What if you're comparing frozen yogurt with ice cream? One-half cup of a typical brand of vanilla ice cream as 137 calories with about 7 grams fat (4.5 of that is saturated). It also has about 16 grams of carbohydrates from the added sugar, 2 grams protein, 84 milligrams calcium and 9 grams magnesium. From a nutritional standpoint, about the only difference is in the amount of fat - there's more fat in the ice cream.
If you're looking for good nutrition and probiotics, the regular yogurt is the best choice, if you're looking for a frozen dessert, then the frozen yogurt may be better than ice cream, but not by much. You also have to keep in mind the serving sizes. If you load up a bowl with either ice cream or frozen yogurt, you're consuming two, three or more servings and the calories, sugar and even the fat are going to add up quickly.
Sources:
Academy of Nutrition and Dietetics. Yogurt Wars: Frozen Vs. Regular. Accessed November 27, 2012. http://www.eatright.org/Public/content.aspx?id=3222.
United States Department of Agriculture - National Nutrient Database for Standard Reference Release 25. Accessed November 27, 2012. http://ndb.nal.usda.gov.