Over here we are more likely to say Oblivion, since it is the Oblivion forum. But I shall try my best to weigh up both.
I personally have the Fallout 3 Game of the Year edition, and to be honest, I really do prefer Oblivion. I just don't like the game in Fallout, I find it too hard and there's no real excitement for me. Everything seems a bit grey and brown in Fallout, and I know it's supposed to be like that, but when you've wandered in the Great Forest or swam in Lake Rumare on Oblivion, nothing else can really compare.
Obviously if you like both games, you need to decide which gives the best extras for the money. I bought Fallout 3 GOTY pre-owned from a shop called Gamestation, and it was fairly cheap, about £8-10 I think. The GOTY version and the normal were the same price, so I went for the GOTY. I didn't think the price for that was bad at all.
You probably won't find Oblivion for the same price. I couldn't get vanilla oblivion for less than £15 in the same shop, and I've just looked and I can't find the 5th Anniversary Edition for less than £20 if you're buying the disk (I don't really know how steam and things work to be honest.)
So to sum up- I prefer Oblivion and I really like it so I wouldn't mind paying a little extra. Have you played both before (even the vanilla games) and if so, which one do you like better?