This is a quality vs. quantity question to me. I own and love both games, but while Oblivion might be a more 'delicate' experience, it just can't match the longevity of WoW. I mean, all these people think 100-200 hours ingame in Oblivion is a lot... I just checked the /played on ONE of my WoW characters, and I was shocked: 23 days and 4 hours (556 hours). And keep in mind, I have a few other characters with a couple of days ingame as well. And I think this is the case for most WoW players (I know most of my friends who play WoW have played a lot more than me

).
One of the most widely used arguments against WoW is the monthly fee. But for me, it's not a problem. I spend, what, 15$ a month on the game, and considering how much time I (and most other WoW players) spend ingame in a month, it suddenly becomes a very cheap game/hobby. A few years back I purchased a game a month because I simply couldn't squeeze more out of a game than one month, and the price of a new game is about 75-100$ here in Denmark, while WoW is ~17$ a month. In that perspective, WoW really is the cheaper option.
But financials put aside, WoW is a very repetitive game in the long run, and I don't see how one could play only that game, especially for extended amounts of time. I myself "supplement" with a few quality gaming experiences, and Oblivion is certainly one of those.
So to sum up, I can't chose one of these games over the other, as the experiences are so different. It's like comparing bungee jumping to watching a movie; the movie is a lot cheaper and lasts a lot longer, but can't compare to the thrill of jumping off a bridge
"If making games is a hobby for you then it's not about the tools you use to make the game, but the games you produce with the tools you have." - Jeku