Are you wanting to write something about the pros and cons of bariatric surgery or do you want to compare and contrast something about the surgery? If you are writing about the pros and cons, you will have to do some research and uncover things such as how successful the surgery is, what are the side effects, are the risks of the surgery worth the benefits? You will have to find some statistics to support each side if you are doing compare/contrast.
There are also different procedures with this type of surgery. Did you know that in one procedure, the stomach size is reduced and in the other procedure, the stomach size is reduced some but the surgery ties off areas to decrease absorption. You could compare/contrast the malabsorbtive and restrictive procedures. Which is better and why? Which results in more weight loss and why? Which is safer and why, etc.
There is a pretty high risk factor in this type of surgery, so maybe you might want to do a compare/contrast from a patient's point of view: if one is morbidly obese, which is worse, to risk this surgery or continue being morbidly obese with all of the health risks that THIS condition entails.