Revolutioner vs Revolutionist: which is better?

If what you are looking for is this sentence "he was a great revolutionist" then the correct way of saying it would be: he was a great revolutionary; " Che Guevara was a great revolutionary."

Revolutionary: a person who either actively participates in, or advocates revolution. Also, when used as an adjective, the term revolutionary refers to something that has a major, sudden impact on society or on some aspect of human endeavor.

We almost always use revolutionary (versus revolutionist or revolutioner, etc.) to name (noun) or describe (adjective) someone who caused, led, supported, or was important to a revolution (major and fundamental change).

From the Corpus of Contemporary American English (COCA):

And here's a young girl on a horse who was a great revolutionary heroine, so you should by all means stop and see that one.

Date: 1994 (19940703); Title: Authors Remember American Women's Historical Landmarks; Source: NPR_Weekend.
Davies, Mark. 2008- The Corpus of Contemporary American English: 450 million words, 1990-present. Available online at https://www.english-corpora.org/coca/.

The Christian ministry has been located on the corridor for 10 years. "King was such a revolutionist," Thomas said. " That's what we're trying to do.

Date: 2000 (20000113); Publication information: CityLife Atlanta (Extra); Title: A stroll down MLK Drive;The road named for the civil rights leader tells a story as it meanders through town; Author: S.A. Reid, Staff; Source: Atlanta Journal Constitution.
(Same source.)

Revolutionary appeared 7,877 times in the COCA, while revolutionist appeared 12 and revolutioner 0.

According to The New Fowler's Modern English Usage-

The form in -ist is first recorded as a noun in the sense 'one who instigates or favours revolution' in 1710 and was the customary word until the mid-i9c.

Since then it has been challenged by revolutionary (the OED entry leads off with a quotation of 1850 from Charles Kingsley's Alton Locke).

Both words are still current. The usual choice is revolutionary.

Modern US examples:

• Songs ... are a resource both for the loyalists defending their country and the revolutionists overthrowing it—Harper's Mag., 1991;
• Oratory and organization were the basic skills of the revolutionist and he had both—J. Pournelle and S. M. Stirling, 1993
• An obituary in The New York Times described her as “an incorrigible revolutionist to the end- NY Times, 2012

Like the word revolve, it's all about turning things around.

Consider the context: did the revolution succeed or fail? In AmE you will see Che Guevara described as a revolutionary far more often than George Washington, who was a founding father, fighter for independence, creator of the nation - a successful revolutionARY.

One interpretation of revolutionIST would be "one who is a specialist in revolutions" which might denote a history professor doing comparative research (e.g. between English, French and American revolutions) or even a mercenary type consulting on how to promote revolution, both meanings carry the implication that the revolutionIST is not personally committed to the revolution i.e. s/he is NOT necessarily a revolutiuonARY.

One would expect RevolutionER to be equal to RevolutionARY, unless revolutioner happens to be a term used within the re-enactor community to distinguish between "revolutioners", "civilists", and soon.