Assasins creed gets boring after a while because u get sick of killing respawning guards that never end, while in olbivion there is no 1 shot kill, but armor, a design face/color thing, while in assasins creed all u have to wear is that white cloak and that ugly face
I myself do love assassin's creed and its extremely addicting-until you beat it... Once you've beaten it, its not really the kind of game that you'd re-play so I'd go with selling it. However, Oblivion is a game with so much to do, almost as if there is no real way to beat all of it. Because of this, Oblivion is much more of the type of game that you could play without getting tired of for quite some time... Based on different opinions, it could go either way, but Oblivion seems to be that kind of game that you could stay with for longer.
Tough choice, can't decide, I have both for 360, and they are my two favorite games. Too hard to pick, but I have beaten Assassin's Creed, and the story is awesome. I have beaten the main quest in oblivion, but not all of the side quests. The main story line in Oblivion is Ok, but the insanity of so much makes it an awesome game. I have spent hours just killing the same person over and over again in Oblivion, and have gone on a murdering spree in Assassin's Creed. It is a tough choice, thagt I cannot decide on.