It's an important difference.
Fox News is a right-wing punditry operation. They spin everything that happens in a light that promotes the viewpoints of US right-wing policy. If right-wingers are in power, they spin to the government's favor, and otherwise spin against the government.
RT is a literal government propaganda outlet. They have a story of what they want to tell people happened (regardless of whether it did or not), and tell people that it happened, to the point of routinely hiring actors as interview subjects. (side note: the Russia media really needs to get a larger acting pool, though... it's funny but sad when the same actor claims to be several different people for different stations in the same week).
If you see something inflamatory claimed on Fox, it's almost certainly spun. Possibly outright false, but unlikely - generally just highly spun. If you see something inflammatory claimed on RT, it's almost certainly false. Possibly just heavily spun, but generally willfully outright false.
Example: Fox News will pick random true stories from around the country, overplay them, and tell you that there's a War on Christmas. RT will hire a woman to play a refugee from Slavyansk to weepingly tell you that the Ukranian army is crucifying children in the town square to torture their mothers before killing them.