Oh God, not another Oblivion vs Morrowind thread... I'm not saying wich one is better than the other, as both are amazing and I love them both for what they are and what experiences can bring me.
Ok, both games are not to be triffeld with, both are HUGE and you will lose hundreds of hours for just exploring and interacting with the NPCs. Oblivion's main story is not the best story you'd ever heard in recent years, but it's quite lenghty and quite fun, while Morrowind's story is even more lenghty and much, much better.
Graphics wise, both games look great in their own respective areas. Oblivion looks more realistic and has lush green forests, lots of wild life and has a very western medieval feeling to it, while Morrowind looks more fantastic with it's creature design, wich is much more varied than Oblivion's. Also, both games use different engines and both have their quirks, first, Oblivion uses the very same engine used on Fallout 3 and New Vegas, so you know what you're dealing with, except it won't freeze to death like New Vegas and Morrowind uses another completely different engine, wich is less bugy and, certainly, not as pretty as Oblivion.
Gameplay wise, well, both have their quirks and kinks. Someone would call Morrowind's combat clunky, even crappy, I call it classic. And Oblivion has a much more streamlined combat, if you're already used to the melee weapons from Fallout 3 and NV, then you're going to feel right at home here.
Then there's the NPC interaction, wich both games are quite different. Oblivion, just like Fallout 3 and NV, will have every single NPC voiced over, but annoyingly, there are like 10 different voices throughout the entire game and you will be hearing the same voices over and over again. Just like the Fallouts, they will have plenty to say and there are going to be lots and lots of different options to interact with them... if you have the money... you see, to make a character spill the beans, you can bribe them and they will "trust you enough", making the speech skill quite useless.
Morrowind is a bit more conservative in this aspect, as not every character is voiced over, just a few words and then you got to read... a lot... but there are WAY much more options to interact with them than in Oblivion and there are many more outcomes of a single conversation than in Oblivion.
If you're going to play it on the PC, I would suggest you buy Morrowind from Steam, as it includes the main game and two expansions, Tribunal, wich adds new quests inside a big city and Bloodmoon wich includes new areas to explore, werewolves and the chance to become a werewolf yourself. Be warned though, that Morrowind is quite an old game and if you're planning to play it on higher resolutions with widescreen aspect, you need to work a little to get it working, but it will be worth your while as it supports widescreen aspect rather nicely.
To get Oblivion, Steam is the way to go too, there are 2 versions of the game, the "cheapest" one includes the Shivering Isles expansion, wich adds a new area to explore and has a bit better story than the main one and The Knights of The Nine, wich is a new quest for goody goody characters. The other version includes those expansions, plus the other DLC, including the infamous Horse Armor, get this if you really want to get everything Oblivion offers, but it's really not necessary as the other one already offers quite a lot for much less.
Both are awesome and if you can, get them both, but I can't really tell you wich one you should choose.