On the surface, the quote may look like an attractive proposition to people who gravitate towards Epicureanism. On closer inspection, the quote is deeply anti-Epicurean, however. It reveals preference of non-existence over existence. Such a sentiment leads to philosophical pessimism where the only sensible course of action is taking Twain's hundred million years holiday prematurely. Out of nowhere, we find ourselves in the world of Arthur Schopenhauer or Philipp Mainländer.
Mark Twain had some 75 years of opportunity to resume his holiday if he so truly desired and yet he didn't take it. He died when opportunity was no longer opportunity but an unavoidable necessity.
I also don't think Twain thought through the nature of non-existence as he seems to romanticise it with the absence of undesirable feelings (worry, grief etc.) and the presence of desirable feelings (content, satisfaction etc.). Non-existence offers nothing.
That said, I also am a huge fan of Mark Twain. He was an exceptional writer.