Less detailed graphics in one big seamless world but with lots of variation a.k.a. World of Warcraft
Definitely this one is the best of the three options, to me! Great graphics are wonderful, don't get me wrong, but gameplay is far more important than visuals.
The 'ideal' would be Dungeon Siege with huge open areas (IE cutting edge graphics, no loading screens) but if that's not possible I'd prefer to lose graphical quality than a sense of immersion. The more 'real' the world feels, the better - and that includes different places looking
like different places.
And with a mini-map to aid navigation! Can't believe I forgot that on my list of things I'd like to see in the game! Huge areas are great: huge areas it's really easy to get lost in are not.