Hreflang-XML sitemap vs HTML vs HTTP header for big site?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Jacek Wieczorek: I always recommend html hreflangs for the reason you pointed in your post. It`s easier to debug the code and find errors in html. Don`t worry about page weight - it`s just a new line in the code.
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  Shao Chieh Lo: Jacek Wieczorek Thank you for your insight 😀
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  Jacek Wieczorek: Well, I meant html rather than http header. Http headers are even harder to find.
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  Shao Chieh Lo: Jacek Wieczorek okay, got it 😀
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  Nathan Nikolay Gaidy: .
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  Bill Hunt: Hi Shao - I have always been pro XML site map for large sites and
  those with many language versions. To use either http or meta tag it would require Google to visit all 10 million of those pages to validate the cross references. Eric Enge has done a number of studies on crawl rates and how long that would take.
  
  With XML Google can get the alternate verifications but ingesting the site maps without the need to visit the pages. This is why we see hreflang issues solved in 48 hours via XML vs weeks for the other two methods.
  
  For QA you can let Google do that for you. The language report is a good indicator of the quality of the XML. If there are few to no errors you are in good shape.
  
  I am trying to finish a guide that deals with these enterprise challenges but solving these problems keeps getting in the way.
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  Shao Chieh Lo: Bill Hunt Thank you so much Bill, this is very insightful, definitely let me know when the guide is available 😀
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  Richard Hearne: Http header versus HTML tags will add very similar weight to each page. HTML tags a little greater of course.
  
  XML sitemap for very large site will probably be easier to maintain and update. It`s far easier to implement any of the choices than maintain them. You always need to consider the dev/production overhead of maintaining this as that`s where the real overhead lies. So you want o consider this holistically - not only what makes your life easier as an SEO.