I don't know either but I recently went with the increased radius. And I don't know if I'm missing much damage from the other morph but I certainly like the bigger circle. I sometimes enjoy throwing encase and dropping the lightning; it's much easier and quicker now with the bigger area.
I can tell you that in all the Repecs of my Sorcerer, I have never put points back into Lightening Splash. There are more effective skills in the Storm Calling line (Mages Fury/Endless Storm/Surge/Bolt Escape). Save your skill points for that.
That being said, more damage is usually better than less, as the mobs tend to come to you so that you need larger areas less than you think.
Enjoy the game, life is what you really want to be worried about.