I'm with Neil on this 100%. If you're primarily a SP kind of guy, you get exclusive episodic content this fall to expand the "campaign". As for MP, there is nothing better than being able to communicate with your team (friends) and really bring things together.
How pointless would Free Roam mode be if you couldn't talk to the people you were running around the city with?
As far as the graphical differences, unless you plan to buy both systems and set them up side by side running both versions at the same time - I don't think you'll notice a thing.