It is odd that you would choose Twain as the contrast to Hemingway. They are, to me, so very similar as to be categorized as the same kind of literature in my head. I enjoy them both, and feel they are excellent purveyors of the craft of the American novel.

There was a great anecdote in one of Asimov's autobiographies about Hemingway, which has stuck with me for years. I am on a tablet at the moment so I won't try to find it, but the gist of it was he went to see an editor when he was quite young and maybe unpublished. The editor asked him, "How do you think Hemingway would say it was raining outside?" And Asimov replied he didn't know. The editor answered, "It was raining outside".

I am pretty sure that koan-like piece of advice shaped Asimov in pretty good ways...and also summarizes Hemingway surprisingly effectively.