If you already got Oblivion then there's no reason not to get morrowind.
There are far too many reasons to count to get Morrowind.
Different pace of gameplay, different (greater) amount of character customization, different rules of the world (like no magicka regeneration and no 1-hour wait to get full HP, the ability to sleep in stolen beds, having to sleep for more than an hour to regenerate fully, etc), more quests, different dungeon styles and different approach to dungeons in general, and many more.
I prefer Morrowind, obviously, but Oblivion has its own merits, just not many that I like myself.
Of course, Shades it going to pop into this thread to say "Morrowind or Oblivion? Daggerfall rulez". Tehe