Meta Robot
11,069,238 websites

Meta Robot is free to use

https://developers.google.com/webmasters/control-crawl-index/docs/robots_meta_tag?hl=en

This page contains a meta robots tag which tells search engines and robots to index or not index the page.

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 X-UA-Compatible is leading in Top 10K Sites, Top 100K Sites and The Entire Web.

2 Meta Robot is leading in Top 1M Sites.

Websites Categories

1 Meta Robot has better usage coverage in more websites categories. Including Arts & Entertainment, Health, Food & Drink, Business & Consumer Services and 10 other categories.

2 X-UA-Compatible is leading in Computers Electronics & Technology, Science & Education, Games, Lifestyle and 6 other categories.

Geography

1 Meta Robot is leading in most countries, including Japan, Germany, United Kingdom, France and 135 other countries.

2 X-UA-Compatible is leading in United States, China, South Korea, Canada and 23 other countries.