AH! I love "mine's bigger than yours" arguments.
You can't really compare 2 games that have yet to come out. But, we take the previous games in the respective series and compare certain aspects of them. as objectively as possible, keeping in mind that one comes from a US powerhouse game developer, and the other from a then-independent Polish studio. Based on that, and assuming the franchises will produce something similar with some improvements, people could draw their on conclusions.
BTW, if you want to take a look at Witcher 1, is on sale for $5 at GOG, and no DRM.
A lot of the points below will answer some of the posts on this thread.
Character build - Witcher is based on a book, with a well-developed character, while the main characters in TES have no real history. Skyrim is going to deviate a bit from that, as the main character is going to be the Dragonborn (and if I am not mistaken, a descendant of the Septim line?) Anyway, this aspect is pretty much a matter of preference. Personally, I have no real preference with either.
Linearity - At their core, all MQs are linear, as you cannot advance to the next stage/chapter without completing the previous step. However, Witcher does give you a wider variety of outcomes from certain aspects of the main quest which affect how the MQ plays as compared to Oblivion. A small example: at one point, you have to conduct an investigation to advance in the MQ. You are given 5 suspects. Depending what you find and what questions you ask, you can end up finding a different character guilty, even fight and kill that character... killing that character or antagonizing that character will have some ramifications later on.
Choices - Oblivion mostly has do/do not choices, and these usually are for side quests. The MQ doesn't give the player any choice. Wicther gives the player 3 distinct paths to the main quest, and depending which path is taken, there will be in-game consequences.
Consequences of choices made - In Oblivion, the player's actions have rarely any considerable consequences to game play, and really none on the MQ. In Witcher, choices made do have consequences on game play and how the story plays, what dialog you get, what cut scenes you get, what side quests you get, etc, Small example: at the very beginning of the game, the player is given a choice to either save a wicthers' laboratory or fight a monster. (MQ) At the time, there's no clue as to what would happen if you decide a or b, but one of those decisions will make the game harder, and you don't find out until you have played the game a lot further.
Open World - Witcher doesn't have an open map. However, the story does account for it and why you can't go freely from section to section. Note on Witcher 2: there are certain areas in Witcher 2 that the player will not be able to reach, depending on choices made during game play.
Exploration - Oblivion obviously has better exploration. The game is meant for it. Witcher isn't meant to be explored.
Meshes/textures (NPCs/weapons/clothing) - The Witcher has better models, but Oblivion has much, much more. Just like DA:O, the main difference among Wicther's NPCs are colors. Witcher's number of weapons pale in comparison with Oblivion's.
Animations - Witcher has better animations all around. Combat alone, there are 6 different styles (7 if you count "non-witcher" weapons) which can be enhanced as the player advances in game. Note on Witcher 2: it has Muay Thai style fist fighting.
Dialog - Witcher can get cheesy, but that is part of the "lore", as in the books on which the game is based.
"A.I" - Note: as someone who codes C, java, perl, shell for a living, I don't like the term "A.I". It doesn't really exist. It is basically a term coined out of sci-fi movies. I prefer the term "coded behavior", which it really is ... but I digress.. both games have some level of coded behavior. However, Witcher has certain touches not seen in Oblivion, like people fishing, or reacting to the rain, reacting to the time of day, reacting to being drunk, to being upset/happy with the main character, etc. It is worth mentioning there are some environmental touches as well, like bats flying out of caves when you enter them, or swallows flying out of ruins, birds flying off the group when you run by them, etc.
Minigames - Witcher has gambling and drinking games. Not really care myself, either way.
Rating - if Witcher doesn't get an AO rating, I'll be surprised
DRM - Witcher's DRM has been removed by a patch. Wicther 2 will have no DRM if bought through GOG. Clear winner here!
Modding - both games can be modded, although Oblivion's construction set, the fact the game uses NIFs and has more vanilla models to work with, makes Oblivion the clear winner here.
For me, and based of what I have seen from the previous installments of each franchise, I have been expecting W2 with more anticipation than Skyrim, because I feel the story is deeper, more complex, and the decisions I make have real consequences in game play. ANd did I mention no DRM?