I think the nomination should go to Kelly.
It doesn't matter all that much if he's not great at debating. His resume, first with the Navy, then with NASA, and then in Congress, speaks for itself. We know how the man-o-sphere, and especially Donald Trump, will react to a man playing second fiddle to a woman leading the ticket. Kelly's backstory - retiring as an astronaut so he could care for his disabled wife, Representative Gabby Giffords, after she was targeted for assassination, then running for Congress so he could continue her work - is pretty compelling as a testament to decency, strength, and respect.
Kelly can directly confront the issue of Trump endorsing political violence. He's also pro-choice, which means he's a good choice when it comes to the two most compelling issues Democrats are focusing on in this election. IMO he's a bit too moderate when it comes to addressing climate change but he does speak openly about it and supports clean energy initiatives.