Canonical Tag
13,171,026 websites

Canonical Tag is free to use

http://en.wikipedia.org/wiki/Canonical_link_element

A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical", or "preferred".

VS

X-UA-Compatible
11,141,235 websites

X-UA-Compatible is free to use

http://blogs.msdn.com/ie/archive/2008/06/10/introducing-ie-emulateie7.aspx

Allows a website to define how a page is rendered in Internet Explorer 8, allowing a website to decide to use IE7 style rendering over IE8 rendering.

Market Share by Top Websites

1 Canonical Tag is leading in Top 10K Sites, Top 100K Sites, Top 1M Sites and The Entire Web.

2 In terms of market share, X-UA-Compatible is clearly lagging behind, losing to Canonical Tag in all segments.

Websites Categories

1 Canonical Tag has better usage coverage in more websites categories. Including Computers Electronics & Technology, Arts & Entertainment, Food & Drink, Games and 15 other categories.

2 X-UA-Compatible is leading in Science & Education, Finance, Law & Government, Jobs & Career and one other category.

Geography

1 Canonical Tag is leading in most countries, including United States, Japan, Germany, United Kingdom and 152 other countries.

2 X-UA-Compatible is leading in China, South Korea, Taiwan, Hong Kong and 6 other countries.