while I found some nuggets here and there in this forum, I chose to open a new thread with a more suitable title.
https://www.rockstargames.com/newswire/article/60459/The-Red-Dead-Redemption-2-Special-Edition-Ultimate-Edition-and-Collect
apart from the differences listed on the official website, I seem to have found these additional ones:
https://fextralife.com/red-dead-redemption-2-animal-parts-selling-guide/
Those possessing the special or ultimate edition of the game can sell parts for 10% more.
https://reddead.fandom.com/wiki/Talismans_%26_Trinkets
https://primewikis.com/guides/rdr2-iguana-scale-trinket/ (link 1)
trinkets in the Special/Ultimate edition:
Eagle Talon Talisman: Raises Eagle Eye duration by five seconds
(Eagle Eye's primary use is to assist the player when they are hunting animals, by highlighting the trail of the target and thus leading the player to it.)
Iguana Scale Trinket: Decreases damage player receives while on horseback by 10%
(link 1: how to equip? : You don’t, because this item is a trinket and as all RDR2 Trinkets, you only have to keep it in your inventory to activate it.)
not sure how factually correct this is, but I'll take his word for it. I couldn't find more resources for this claim:
https://gaming.stackexchange.com/questions/340346/war-horse-died-possible-to-get-it-back
When it comes to the War Horse, Dappled Black Thoroughbred and Black Chestnut Thoroughbred, the "Special Edition Red Dead Redemption 2 Official Strategy Guide" says:
Note that horses exclusive to the pre-order, Special and Ultimate editions of the game are unique in that they respawn at the Valentine stable when they die.
one thing you won't be getting is:
"Treasure Map: Secret buried treasure can be found throughout the world of Red Dead Redemption 2. This exclusive Treasure Map for Story Mode will give Arthur a head start on the hunt for hidden riches. Must pre-order by August 15, 2018."
https://www.rockstargames.com/newswire/article/60459/The-Red-Dead-Redemption-2-Special-Edition-Ultimate-Edition-and-Collect
via Kormath
https://www.rdr2.org/forums/topic/4457-standard-and-special-edt/?do=findComment&comment=32708
differences standard edition special edition
differences standard edition ultimate edition