Admittedly while both are compared to Rome and the RU definitely likes to ape the Roman Imagery. The USA is actually a far truer successor to Rome when you get down to it.
Considering while both did massive conquests of Latin America the USA took a much more gradual, careful approach to it. Preferring to assimilate conquered peoples from the top down and adopting useful ideas from them. Over the horrendously costly and genocidal approach of the RU. Alongside a preference to religious tolerance of other faiths so long as it doesn't threaten the establishment. Not to mention the large scale use of slavery as an institution. Overall I want to the USA from Decades of Darkness is much more pragmatic evil/even evil has standards when compared to the RU.
Honestly I see the USA from Decades of Darkness winning in the long term over the RU in a fusion timeline. (Also add Dystopic Return of Magic, Separated at Birth, Swarm on the Somme and Green Antarctica into the mix).