To start off with, diet or lower fat percentage products have been adjusted so they have less calories, sugar or fats. This is for those who are calorie conscious and wish to lower their body weight or prevent excess fat gain. Often this is accomplished especially in drinks, by removing the sugar and substituting it with a chemical alternative. An example of this would be aspartame, which can be found in almost all sugar free or reduced sugar drinks. So what is the harm? They have been linked to increasing the likelihood of you developing certain cancers.
The low fat foods such as butter have also been chemically changed to reduce it. Butter in its natural form is ‘x’ amount of calories so to change it requires specific processes which can be adverse to your health. A general rule is to eat a Neanderthal diet, which briefly is eating the foods that we have eaten for the past few million years or so, avoiding packaged foods and eating natural. Modified foods are, like sweeteners, not good for the body and can have adverse effects over long term consumption.
So then you would expect the answer to be, eat natural and eat full fat! But then of course you have the sugars, which cause insulin spikes adding to weight gain, calories which increase weight and a higher level of tooth decay. Food that is full fat also has a greater number of calories adding to increased stomach fat and obesity related diseases such as heart disease.
So the answer then is which do you prefer, possible increase chance of cancer or obesity? Personally I stick to drinks low in calories but will eat full fat foods, I would not worry about it too much as everything these days is linked to cancer and obesity. Just take this into consideration and implement it into your diet and lifestyle.