Groundslam vs Earthshatter vs Sunder

Heya,

I levelled the campaign with Sunder, and then switched to Earthshatter for maps. I am going ok, but wondered what the main difference between specifically groundslam and Earthshatter is (yes, I can test, but am at work and trying to theory craft some stuff).

I see alot of the top players have changed from Earthshatter to Groundslam (of Earthshaking) and was wondering why? Is it better single target and they are going for the uber kills etc?