Personally, I have never, not once, felt a need to pick one over the other. I don't think of the MW while playing OB and I don't think of OB while playing MW. Both have their strengths, I have played both for hundreds upon hundreds of hours and both are still installed and still played to the present day.
I feel it's important to discuss the merits of one game over the other because it allows us to both understand what features were would all like most and what we wish TES V to resemble. The complaints about Oblivion surely shaped Fallout 3, and the complaints about both will positively shape how TES V ends up.
As for right now, I must say Morrowind was my favorite, and that Oblivion, while a good game, was a disappointment as a TES title. It's not so much the game mechanics (which were a huge complaint, because I never, not once, felt like I was making progress as a character, due to the level scaling), but rather the presentation of the game world. No religious factions, no army factions, the downsizing, and the player coddling. It just really messed up the feeling of the world for me.
Luckily Todd Howard recognized the problems himself, such as when he mentioned how he wanted their next TES game to be much more rich in the lore and culture.