I'm pretty sure it's an all or nothing thing. Either you're in a sanctuary and get rest XP or you're not and don't. I don't think there are any different tiers in how fast it accumulates.

(When the game first released, there was an advantage to logging out at the inn because the main cities were not at the time treated as sanctuaries, except within the Inn. You got rest XP if you logged out at the inn or around an outpost's Aetherite. But it didn't really make a lot of sense why the outposts were safer than the cities, so they changed it to make the entirety of each of the main cities act as Sanctuary. Since that change, it hasn't really mattered if you go to the Inn or not. I still normally do, but that's mostly an immersion thing.)