Agreed with all of this.
I usually enjoy Lara (or at least tolerate her) when she does the human interest/fluff stories, but when she fills in for Robin, I'm really turned off. She just can't do hard news (maybe it's the perception of her in general and her work).
She was just really great when with Josh and Sam. Their chemistry (with and without Robin & George) was spectacular and couldn't be broken with a sledgehammer. It was one of the reasons I watched GMA during that era and was a regular fan of their after-show on Yahoo.
On Amy, she's great doing whatever you give her. She already proved herself as news reader, but really shined when she also fills in for Robin (perfect chemistry with) and does just as amazing with lighter stories.
I generally like Lara too, and I agree that while she's great with what she does best, Amy is great at it all and should get paired with Robin and George more than her.