Which is the proper spelling: "Adapter" or "adaptor"?

The "or" suffix tends to be more common in American English. In English English, the "er" prefix is usually preferred when not referring to a person. In this instance I would certainly use Adapter.

I tend to agree with all the talented linguists that the suffixes are used interchangeably; however, from my modest experience, the -or is used more frequently for people while the -er, for things/items. I do not have enough official evidence either to prove or disprove the fact that one is used more in the US and the other in the UK, but it is all possible... since we have other influences on the living language; technology, environment, globalization, robotics, etc. are all creating new versions and meanings we may not have heard before and thus, need to be open to those usages as the language - I would even attempt to say, the new language - is being expended and created not only in the textbooks but in various genre of literature. Thank you.
Dr. M.

I would tend to suggest in a situation like this where both spellings are considered acceptable to use the spelling that is most broadly accepted in the particular field you are working in. In this case, the leading example of usage of the word "adapter" is the description of the kind of system you are writing in Design Patterns: Elements of Reusable Object-Oriented Software by Gamma, Helms, Johnson and Vlissides, commonly referred to as "Gang of Four" or "GoF". This book (which is among the most widely-read books on software design there are) uses the -er spelling, so I would suggest that anybody else working on software adapters should also use that spelling.

Here's a good article on the subject... essentially, there is no 'correct' answer to this question.

Incidentally, some would argue quite the opposite in regards to either form referring to people or objects. There are (not literally of course) endless examples of people roles ending in -er. Caterer, lecturer, carpenter etc.

The British newspaper The Guardian, for instance, makes a distinction between adaptor vs. adapter in its internal style guide, namely, use adaptor when referring to devices and adapter when referring to people.