Difference between "commentor" and "commentator"

Solution 1:

One difference is that as far as Merriam-Webster is concerned, the word "commentor" does not even exist. Same goes for "commenter".

I do see the word "commenter" (but not "commentor") being used on Reddit, blogs, or actually right here on Stack Exchange. Others have pointed out that it seems to be an online thing, and I agree.

However, I disagree with the people saying that "commentator" sounds like someone who comments on sports. There are, for example, political commentators.

Back to Merriam-Webster, my understanding is that they are usually not too slow to catch up with the latest trends. They have a special section "New Words & Slang", a collection of user-submitted words. There you can find, for example, not one but five proposed meanings of the verb "to facebook". However, even that dedicated section doesn't mention "commenter", although blogs and online forums predate Facebook.

All that being said, there is no question that morphologically, "commenter" makes perfect sense. A killer is someone who kills, a driver is someone who drives, and a commenter is someone who comments. The "-er" is a so-called agent noun suffix, and it is very common in English.

On the other hand, the agent noun suffix "-or", while it does exist, is not common at all. Wiktionary lists as few as twelve terms that were derived using this suffix, and offers the following usage notes:

English generally appends this suffix where Latin would do it—to the root of a Latin-type perfect passive participle. For other words, English tends to use the suffix -er. Occasionally both are used (computer vs. computor).

Depending on where you live, you may have never encountered the word "computor" at all. However, I bet you have seen the word "computer" many times.

The bottom line:

1. The word "commenter" is pretty common online, so if you use it, people should understand you. In formal writing, you might wish to use "commentator" instead.
2. If you want to be on the safe side, you probably should not use "commentor" at all, though nobody can forbid you to do so.

Of course, the English language is constantly evolving, so both of the above recommendations may become obsolete as time passes.

Solution 2:

A commentator is someone who commentates. A commenter is someone who comments.

A commentary would consist of many comments, normally as an event unfolds (be it sports, political, etc.) It would also be valid to have a commentary on a written text which may be explanatory notes, etc.