I think he would have, after a long debate with himself, just to make sure no one else died after him. But he would have been willing to die to save Layla. He's that giving.

I'm not sure. A part of him knew that Sue Ann had to be stopped but at the same time he wants to save lives. And if his life was the one to be exchanged for Layla's that would mean no more civilians would get hurt. At least this time. And I think he'd trust that Sam would make sure there were no other times. When the Reaper came for him, he didn't even try to run. I think a part of him was hoping that Layla could still be saved. He risks his life to save complete strangers every day. In his mind, was this any different? The only thing that might hold him back is the idea that he'd be leaving Sammy to fend for himself.

^^I agree. I think it would have been a really tough decision for Dean to make. Like fanfly said, he risks his life for strangers everyday so that is nothing new to him, and this woman, Layla, she actually made an impression on him. He cared about her, and as we know with Dean when he cares at all, he cares a lot, so that would have been a hard choice to make for him, choosing his life over this woman, who he felt deserved life more than he did. I think he probably would have been more than willingly to let her live if not for Sammy. It would have been hard for him to leave Sammy to fend for himself, so like I said, I don't know what would have happened. I feel he would have been really torn about it.