On the surface Natal seems to be more integrated, like being able to use it to navigate the dashboard, for example. they seem to have hinted at that pretty strongly in the past. Arc seems like an add-on, versus part of the system experience itself. So it makes sense that Natal would A. be bundled and B. sell more units.
I, personally, am looking forward to neither. I hate Wiimotes in almost every game I am forced to use them. Though I do like Sports Resort archery, for whatever reason, even though I am terrible at it. :)
My money's on both of them kind of sucking, especially since MS nerfed the Natal to get it to a reasonable price.