Short answer: depends.
As you can see you already have different, even contradictory answer after only a couple of posts. Some time ago, and even today on lower levels, some teams (still) play(ed) left/right centres as opposed to inside and outside. Not the norm, but it happens. It depends on a lot of circumstantial factors.
My take, based on a couple of facts and lots of personal opinions. Fact: on average, Outside tend to have more space than inside centres. This has implications both on offence and defence.
I agree with most of what unrated said here. Physically, I would say on average IC are stronger, more tankish and OC more elusive and fast. IC can be playmakers but also space creators. IC tend to have a lot of work defensively as a) they tend to be pushed a lot and b) they have the incentive to cut the opponent fast as this would stop/disrupt the flow on the opposing team's play.
Having said that, and as physically demanding as the IC's defensive task is, i would go on a limb here and say that from a decision-making point of view, the OC's defence is the hardest job on the field. He has too many variables to consider and given his position, every decision is do or die. The options at that point are not that far from endless and he needs to decide on his feet.
On average (again) i would say IC have to be tidier in the sense of following the play and OC's have more leeway to do what they see fit once they get the ball.