I can't decide between Assassin's Creed and Oblivion!11!1!!1!

My head is gonna asplode! They are both 30 bucks, and I'm low on cash so I can only get one. I like the lengthyness and the rpg aspect of Oblivion, but I also like the action and the assassinations of AC!1!!1!!1!!! I don't know what to do, I can't make this decision by myself lol, which one should I get, and please give reasons..

EDIT: I added a poll to make things easier.

If you're looking for only one and longevity, I'd recommend to buy Oblivion and rent Assassins Creed as it's also a good game.

I personally own both and have over 50 hours of gameplay in AC, but in comparison to Oblivion it's not a contest.

Both are great games.

Oblivion. I don't even like that type of game and I'd still recommend it over Assassin's Creed. Assassin's Creed is the definition of monotony.