Search engine spellings?

[wright_david1]

To very broadly answer your question at first, the search volume in the US is larger than in the UK so if that were the only factor, the US spelling would dominate.

That's not the only factor, because search engines do try to deliver results at least somewhat based on your location. For example, if you're in London and searching for "Carpenter", you'll get more London-based carpenters in your results. If you're in Manchester and searching for "London carpenter" you'll see results around London.

Since Google and the other major search engines are striving to deliver more personalized results, if you're in the UK and hoping to attract UK-based visitors, "favour" would be more relevant.

Go with the spelling used by your target market. If you're a local business such as a doctor, you'll want to be found by people near your office. If you're selling items worldwide, you may be better off using the more common spelling - though that may be much more competitive and harder to rank highly for that term.