I don't know how opposite Sam's reaction was to Dean's, though. I could be remembering this completely wrong (it's been a while since I've seen the ep) but didn't Sam become obsessed with finding the Trickster so that he could get Dean back? Or was it just to kill him?

You're right in that Sam did keep a much leveler head about it than Dean; he was much more John-like (scarier than John, really; I guess Sam and Dean kept him human to a degree), which actually makes me wonder whether Dean's sacrifice wasn't much more Mary-like and if that's what we're going to find out about her.

Either way, interesting Meta comparison here. You wonder whether there isn't any resentment on Dean's part, actually, in that he knows that Sam is kind of everything he wants to be and isn't; cool, logical, and able to detach. You get the feeling that no matter how much Dean wants to give up hunting, he wouldn't ever be able to because he'd never be able to stop thinking about all the people he'd be letting down, he's trapped that way. But Sam's not and Dean knows it, and that's at least part of how he justified making the sacrifice he did. That was actually part of how I interpreted Sam's dream about Bela in DaLD; she's the escape he has made once and maybe wishes he could make again.