Is it about "fat" or "calories" in foods??

Fats are calories so your question is a little muddled.

from a strictly body composition standpoint, studies seem to show that there would be no difference.Although that doesn't take into account long term heath, which is the main concern of most people here.

While 50 grams from sugar is essentially the same from an apple or a cake, it's obvious which is a better food choice long term.

Fats are different as there are different type of fat, 50 grams of saturated fat will affect the body in a different way than 50 grams of trans fat.

Just use some common sense when it comes to food choices.

I go by the 80/20 rule, 80% of calories from whole foods and so called clean sources (which again is up for interpretation) then do whatever you want with the other 20%, as long as it's still within your macros it will make no difference and helps keep you sane.

If you are 10 apples you probably would be less hungry after an hour or so than you would eating one cake. This is because apples also contain a lot of fibre which will fill you up and the apples will be digested more slowly than the cake. In terms of losing weight, using filling foods such as fruits, vegetables and lean protein especially, will help you stick to your calorie allowance more easily without feeling hungry

According to science, if you're not talking about satiety or general health, weight loss is about calories in vs. calories out. One lb. = around 3500 calories. Not fat grams, not sugar.

I am no expert but like to read nutritional books etc and have done some A Level Biology and Chemistry in the past. from what i recall, all foods are essentially broken down into their basic food groups during digestion: Macro nutrients: Fat (lipids), Protein (amino Acids) and carbohydrates (sugar- eg from strachs such as pasta and bread or fruit etc) Then you have waste-fibre helping to pass things along....Ahem! and finally your Micro nutrients (Vitamins, minerals etc)

Fruit has its own form of natuarally occuring sugar called Fructose and this is more complex sugar (AKA carbohydrate) than the glucose (the type of sugar added to cakes) this means that it will take longer to break down during digestion.
All sugar, regardless of complexity, will break down to its simplest form-glucose, before it can be uptaken by cells from the blood. Take a look at the Glycemic Index: http://en.wikipedia.org/wiki/Glycemic_index or in any nutritional book.
Most will advise that the longer the body takes to break a food down, the longer it will satiate you and therefore encourage you to eat less. Foods with a high degree of protein will also fill you as protein is very large molecule and takes the longest to break down into its constituent parts-Amino acids. I would always eat clean- you litterally are what you eat. Your food is uptaken by your cells for basic function-you cannot live without fat for example as every single one of your cells is surronded by lipo-proteins which control the uptake of nutrients. but if you make good clean choices, your body can utilise it, if bad choices this will reflect in poor concentration, bad skin, health and toxin build up.

Ok i think ive gone overboard with my enthusiasm here. I love reading about this stuff-you can wake up now, i've finshed-promise!!.