Pay down my mortgage or invest?

For my records, and to tackle this question, I have found the following articles to help answer it.  Note: Unless you can predict the future...there is no "right" answer until time tells us so.

1. Afford Anything's take on the matter:  http://affordanything.com/2013/03/22/pay-off-your-mortgage-or-invest-in-the-stock-market
2. Investopedia, "Why Leveraged Investments Sink." http://www.investopedia.com/articles/optioninvestor/07/leverage_underwater.asp
3. Mr. Money Moustache - http://www.mrmoneymustache.com/2012/02/24/pay-down-the-mortgage-or-invest-more-a-winwin-question/

I will have more comments on this in the future, though I'm not sure how much I can add to these great articles.  More than anything, this post is to help me build a resource center for this question.