Report compares 'light' to standard products

 A Which report found that there was little difference in calories between products labelled light and standard. 

The Which? report took a sample of 12 low fat, reduced fat and light foods and compared them to the standard products, only to find small differences in calorie content.. A Standard Mc Vitie's chocolate digestive had 85kcal, and a light one had 77kcal. A Tesco low fat yogurt had more calories per pot at 130kcal and more sugar at 20.2g than a standard Activia at 123kcal. 
To have the word 'light' it must contain at least 30% less fat than the standard version.
The best way to compare products is to see how much they give you per 100g, see how much your portion is going to give and if that amount is something you are concerned about. 
For example chocolate digestives were found to have just 8kcal less in energy than the standard version. This is a treat food and having just one or two, these small differences in calories wont make a difference. So the verdict on this one is to go by whichever you like the taste of. 
For cheese the lighter one has about 40% less fat, which has resulted in the protein and calcium going up - a good choice. 
For the yogurts you will need to compare per 100g too see how much sugar, fat and calories they have and then see how much that adds up to for your portion. Sometimes the lighter product may have more calories and sugar just because you are having a double size portion.
You'll need to weigh up what your priority is and choose based on your needs. If it's taste - go for whichever you like and have a small portion. If it's quantity that drives you and you know you are going to be eating a lot, then look for the one with least calories per 100g.