I think what he meant was
1. No flame wars in the comments
2. Be able to support your choice with good arguments
3. Graphics can't be used as an argument
4. The respective leveling systems can't be used as arguments
5. You have to have played both games to participate in the discussion
That's my best guess.
I was one of the unfortunate many who discovered Skyrim, and then had to backtrack to Oblivion and I could not get into it. I really want to try Morrowind, but I fear the same thing will hold true. Obviously, the combat mechanics have gotten more and more fleshed out over the years and the further back you go the more and more you feel like you're swinging foam swords at a larp event instead of landing heavy, or not so heavy blows, with dynamic results.
That said, I wouldn't mind getting a PC and the new Anthology set. But Skyrim is where it's at and is, in my opinion, the best game out there for vanilla content, moddability (even compared to Half Life), and all you lore nerds. SO much lore.
I'm actually replaying Oblivion as we speak so I should have some good reasons later on. Right of the bat I think I prefer Skyrim, just because the AIs were a bit cleverer and the animations (Not the graphics) looked a bit more fluid. I liked the way casting spells was done in Skyrim as well, I felt more immersed to see the spell being controlled in my hand, but in Oblivion I could have a sword, shield AND a spell to use seemlessly in combat. As I said, I'll probably be back later with better reasons .
Honestly, it's difficult to compare the two. Oblivion is a deep, involved game with interesting quests, dialogue, and a very colorful and rich environment. As far as games go, I'd have to go with Oblivion. However, when you're talking about Elder Scrolls, it's important to note that they're not just a game. It's about the world.
Skyrim is a huge, interactive game world. I could never get myself lost exploring the Cyrodillic plains like I can the frozen tundra of Skyrim. The world is full of wildlife. There's cooking and garden growing. You can have a family. The two are very different experiences.