I had fairly recently read An American Tragedy and picked up Sister Carrie more or less randomly when I was in New York earlier this year. With the latter being Theodore Dreiser's first novel (published in 1900 and thus 25 years earlier than the former) differences abound of course. Dreiser's naturalist and descriptive style - true very much to the novel of the 19th century and not the post-modern style emerging soon with writers such as Joyce or the great Döblin I recently discovered - is the same in both books. It makes for easy reading as it requires little concentration since everything that happens is explicitly spelled out for the reader.
The subject matter is quite different though, or rather the result of the urban American way of living the author describes is quite different. In both novels a young character tries to make her (his) way in an urban setting. Here it is Sister Carrie trying to get a break on her own first in Chicago then in New York. And that describes aptly the scandal of the novel, on her own. Carrie effectively makes her way from one man to another (not too many really, but still), climbing the social ladder in the process until she is at its top on her own. It is undoubtedly a testament to the radical changing of morals that what feels like a perfectly normal behaviour to me (a woman living with one man, then another, then alone) was considered scandalous, even outrageous, at the time the book came out. To go back to Joyce (or Flannery O'Connor, same thing really) once more, where I as the (post-)modern reader have a hard time understanding their (at times) obsession with religious questions, the same holds true for this barriers breaking novel and its protagonist. In the end I enjoyed reading it but it's inner spirit, its fight, is very much a reflection of the conservative culture of the time, one that I note with mild interest but one that I hardly see reflected in my own life (and, less so, times) anymore.