I think a Soap's impact depends on many things; the stories, the actors, certainly the media and most importantly the viewers. If the media hype up a story, and its performed well then its is certainly primed for a larger impact. But it is how we the public take to a story that determines the impact.
I suppose Eastenders generally have a greater tendency to create a larger impact because the audience, (by and large) are slightly younger and will get more excited over the latest thing going on. That doesn't mean that its stories will always have a larger impact or that their stories are automatically more interesting. At the time when EE was in its latest massive high, and especially during the "!everyone's talking about it" time its stories were having a massive impact. It was well written, well produced and the plot holes at that time were much much smaller.
Some of Corrie's stories have also had a massive impact however, Hilman was huge, 1983's love triangle was tremendous. Believe it or not both Sarah's pregnancy and Alma's death were great topics of debate. Sometime stories have the opposite effect but still have a great impact. Charlie Shelley this summer was almost universally disliked, but yet it still made an impact on the audience. I guess this is following Oscar Wilde when he said the only thing worse than being talked about is not being talked about.
Emmerdale's plane crash, or recent storms episodes, JR's shooting, Harold's death/disappearance, Brookside's siege or patio/body story. Stories have an impact if it catches the public imagination, which admittedly is controlled slightly by the media.
I'd imagine EE generally does have a slightly bigger impact due to the type of people that tend to watch it. I think impact however relies more on the actual story and not necessarily what channel it is on