Red Dead Redemption 2 has been out on PC for almost a month now. Released on the 5th of November, RDR2 went through some teething issues with crashes and other errors, but Rockstar has spent the better part of the past 3 weeks fixing everything that was wrong.

Soon, the game rides onto Steam, which remains the largest digital video game storefront.
Initially released via the Rockstar Games Launcher and then through select retailers and the Epic Games Store, Red Dead Redemption 2 will launch on Steam on the 5th of December, a full month after the port first became available. This is no doubt part of a deal Rockstar struck with Epic Games.
Epic has been trying to break into the digital distribution market with their own, somewhat under-equipped store, by offering publishers much better deals than Valve's Steam does. As more and more titles become exclusive to the Epic Games Store, usually for a limited time, a bit of controversy arose too - because competition and developers being paid more is bad, apparently.
The Steam version of the game will still require the use of the Rockstar Games Launcher, so in essence, there isn't any difference beyond which particular hoops you need to jump through before you can start playing the actual game. Since modding Red Dead Redemption 2 is finicky, coupled with the launcher, don't expect Steam Workshop support.
With the multitude of patches that Rockstar has recently pumped out, Red Dead Redemption 2 should be more than stable on PC at this point. Whether you're going to play it through Steam from the 5th of December or are playing it via another platform, there shouldn't be any more performance issues or crashes to worry about.
While Steam's holiday sale is undoubtedly around the corner, don't expect a freshly minted AAA title to go on too steep a discount!