Family is the most important thing to Dean. I think he would've stayed to watch them. Sam couldn't have gone very far knowing that Dean was still alive, and as long as Sam lived, Dean would've been there to watch him.

I really think he would've have gone with her. The idea that he might one day become violent and hurt innocent people? Also, watching his family without being able to help them? Dean at this point is accepting of his own mortality. It's the thought that Sam might be vulnerable without him that keeps him holding on. How much good could he do Sam if he held on? And one day, Sam will die and Dean will be stuck forever as a spirit- alone.

You have a knack for making me change my mind, or nearly so, anyway. lol

I agree with fanfly! eventually after enormous battles with himself,he would have chosen to go!

He was literally two seconds away from going with her.

He was about to let go. I think, like fanfly said, the idea of becoming the thing that he had spent his entire life hunting would have been too much for him. He didn't want to leave Sammy/John, but he would have if he thought it was really his only choice. And I don't really see Dean willing to accept a life of hanging in the shadows, watching his family continue on without him but without having any way to help them whatsoever, so as much as he loves his family, I think he would have left with Tessa, in fact, because it was the right thing to do for the family he loves so much.