I'm making a few assumptions in my answer that I think you'll find acceptable. 1) I'm assuming that second game is Deadspace, grotesque alien horror, really violent. Yeah. 2) I'm assuming that this is a no extra-life no save and replay situation. I die, i die. 3) I'm assuming that this is a real life representation of these worlds. Living in them as if they were reality as the developers imagined them.
That said, 24 hours in Deadspace. I'll take my chances. And let me be clear, my chances of survival are next to nothing, but if I do survive my necromorph disaster, there are parts of the Deadspace universe where life continues essentially normally. Even a high-tech enjoyable life of luxury in a futuristic society. Those are not the experiences of protagonist Isaac Clark, but I could hope that I could maneuver my way around the terrifying man-hunting alien virus and evil cultists and find a spaceship to something better. If not, 24 hours might be plausible to wait it out.
It would be far easier to survive the Fallout universe but less enjoyable to do so and not for eight fucking years. Christ, it's still a total post-apocalyptic wasteland infested with radioactive scorpions the size of buses, fire breathing ants, mutants who carry sledgehammers and say things like, "I will wear your bones around my neck," and cannibals. Something would get me eventually. And even if it didn't, life for those years would be terrible.
Plus the girls in the Deadspace universe seem more attractive. Well... at least until they sprout an extra row of teeth and tentacular razor swords from their shoulder blades.