Well, I don't know exaclty, but they probably pay more. As far as ad format size, I think the bigger rectangles pay more (starting at the 250x250 square to the large rectangle). I had a 200x200 square, switched it with a 300x250 and my revenue jumped. I'm gettin same clicks (maybe a few more with bigger ad), but the cpc is high(er). I'm redesigning website to implement two rectangles in there instead of banner and leader. Will be interesting to see the result.
IMO... its 1 ads. and still based on keyword bidding price. i more prefer text based, not banner ads.