I personally prefer Oblivion more. This is because, not only does Oblivion have better graphics, but Oblivion also has the lines scripted out, and has the lines actually spoken. Also, Oblivion is based in the Imperial City, which looks much brighter, cheerier and more friendly than Morrowind, and the graphics of Oblivion are better overall. Also, Oblivion is much easier than Morrowind was: in Morrowind, it was difficult to even kill a small creature, whereas Oblivion has a difficulty setting, so if you set it on the easiest setting you could complete the game in a matter of days.
However, I liked the fact that Morrowind had a bigger environment than Oblivion did. Also, in Morrowind you could kill pretty much anybody and if you killed a quest character it would say the world was doomed. Whereas in Oblivion you are not actually allowed to kill them. I think the system in Morrowind (concerning killing people) was much preferrable, and added more to the choice element.
However, I liked the fact that Morrowind had a bigger environment than Oblivion did. Also, in Morrowind you could kill pretty much anybody and if you killed a quest character it would say the world was doomed. Whereas in Oblivion you are not actually allowed to kill them. I think the system in Morrowind (concerning killing people) was much preferrable, and added more to the choice element.