Hmmm, well Amsterdam totally fucking rocks its by far my favorite place on Earth. Not just Amsterdam i love the whole country. Although iv never been to Nimbin so i cant give you any insight to what that place is all about, tho i have heard good things. If i were you I would not even think twice it would be the Netherlands in a heart beat. I went for my 18th and again for my 20th B-days and i will Never forget the experience despite the fact i probably smoked about 3 zips if not more worth of material on each ocasion. And make sure if you do pick Adam that you stay in a hotel in the Joordan district, best area by far to stay in(Least touristy area of the city, and one of the cleanist) and make sure u make time to visit one of the many museums as they are truly world fukin class.