Hi and welcome!
@Clematis already beat me to the most important point, that being that your daughter should be, needs to be, actively involved in the decision process. It's wonderful that you are here researching on her behalf, and giving her this gift which will do so much for her. But the best way for her to learn is to do so first hand, and while she is young, she is an adult and old enough to read and learn here, and even participate if she is so inclined. We would welcome her here.
I don't think 18 is too young for bariatric surgery, but it depends on the individual. Some people are mature and responsible enough to handle the DS by 14 or 15, others never get there (trust me, we've seen that in folks far older than your daughter). As far as which operation goes, you know her and we don't, and hopefully she is sufficiently self aware and honest with herself to determine whether or not she's ready to take on the lifetime nutritional needs of the DS. I hope she is, as the Ds will serve her well for much longer than any other operation, and is easier to live with in terms of what foods she can eat, cooking methods, etc. But if she can't, or won't, take the vitamin, mineral, and protein requirements as a crucial lifetime commitment, then sleeve would be the way to go. She would still need some vitamins even with sleeve, and would still need to emphasize protein, but she would be much less likely to get into nutritional trouble. The downside of the sleeve is that it requires a different commitment - to a low calorie, low carb, low fat diet for life. So, which commitment is she ready for?