Oblivion has always been on a higher scale than Fallout 3 for the simple reason that Oblivion, where there is sometimes a threat of danger, you can also relax when you play.
Although the scenery can be quite samey a lot of the time, just passing through a quiet field and maybe stumbling upon a village of friendly villages can be a pleasant experience.
I can overlook the fact that a lot of this was quite potentially copied and pasted for the fact that the game was teeming from every crack and orifice with lore, tomes full of little stories within stories, excruciating attention to detail etc.
Although i never felt myself really get immersed in the game, and i was always one of those "Fast-Travel is the way to go" kind of people(like Oblivion was a part of my hectic schedule and i had to rush through it after lunch to avoid it cutting into my other appointment) I can actually see where people are coming from when they tell me how good the game is.
As for Fallout, when you finish the game, you're screwed unless you buy the addons, the landscaping is ugly, like they were designing a map of Chernobyl and got carried away and they really want to make you feel like there's danger around every corner - Because there bloody well is. If you stand still for too long a mole bites your feet from under you, if you run around to much a DeathClaw rips your titties off with it's massive claws, if you're just minding your business a slaver could run up to you strapped with enough C4 to rip the galaxy a new intergalactic arsehole and blow you away. It just doesn't do it for me.
Bloody hell i can ramble