And here's where, at the risk of sounding like a not very nice person, I have to admit that my first reaction, at least, is to viscerally dislike this scenario and to strongly prefer what actually happened on the show. Because on the show, neither couple got a happy ending, unless you count Robin and Marian cavorting in the afterlife (IMO, ambiguous enough that it can be considered Robin's dying hallucination). You could say that both 'ships lost ... but a 'ship with a tragic ending is still a 'ship. My gut reaction is to agree with a friend (who may or not want to post here) who feels that the "noble sacrifice" scenario would have been an active negation of the G/M ship (and an affirmation of R/M). It doesn't seem a win/win scenario so much as a win for R/M and a "there there, at least Guy gets to be a good man!" sop for G/M shippers. (To be honest, from the "hey, that's my ship!" POV -- I'm not even that happy about Guy's "she was always yours" in his death scene.)
Another factor is that I really don't see anything but a very bleak existence for Guy after his noble sacrifice -- while I've read somewhat plausible Guy/OC fics, he strikes me as fundamentally a one-woman man (Meg, in my rather strong opinion, touches his heart only because she reminds him of Marian). Admittedly, Guy spends most of Season 3 in hell, and hopefully he would be happier knowing that Marian is alive and with Robin (and that he has her friendship and respect) than knowing that Marian is dead at his hands, and that her last words to him were "I love Robin Hood." Buuuut ... no one else gets a happy ending, either. And while I hope that doesn't make Guy feel better, it makes me feel better, to paraphrase something my aforementioned friend has said. (No, I'm not that petty in real life, but these are fictional characters.) Robin and Marian living happily ever after while Guy is slowly dying of a broken heart, with Marian's friendship and respect as a consolation, is a seriously depressing scenario to me as a Guy fan. (FYI -- Tom, in The Man Who Shot Liberty Valance, saves his rival's life for the sake of the girl he loves and then spends the remainder of his days drinking himself to death.) Obviously my opinion is also colored by my belief that in spite of all the bad things he did, Guy's love for Marian was ultimately far stronger than Robin's. (And by the way -- I'm honestly not a Robin-hater, and I don't wish a lifetime of a broken heart on him either, but I believe that Robin would be much more capable of "moving on." Hopefully not with Kate, however.)
Now... in terms of "which 'ship wins," obviously, you could argue that what actually happened on the show is also a "victory" of R/M over G/M. Marian did get engaged to Robin, she did spend her last moments with Guy shouting at him that she would rather die than marry him, that she's going to marry Robin Hood and that she loves Robin Hood. And she did exchange wedding vows with Robin before she died. And actually when I first saw the YouTube clip of Marian's stabbing and death (before my involvement in the fandom), I thought that the show had decisively gone with R/M and destroyed G/M, despite the lack of a happy ending for R/M. However ... having actually watched 2x12 and 2x13, and the several episodes before that, I don't believe that. (Check out this wonderful analysis by dollsome). At least to me, it's pretty clear that Marian was seriously torn between Robin and Guy toward the end, that on some level she didn't want to commit herself to anyone because it was her freedom and independence she cherished most, and that her decisive choice of Robin at the end was due in large part to a tragic set of circumstances -- including Guy's own very misguided actions -- that basically made Guy irredeemable in her eyes, at that point. (I also think that her taunting of Guy in that fateful scene was due at least in part to her anger at him for betraying her to the Sheriff.) IMO, it does not mean that Marian never had any real feelings for Guy. Marian choosing Robin over Guy when Guy has proven himself to be the good and noble man she has inspired him to be would be a far more bitter pill for me as a G/M shipper to swallow. Doesn't mean I won't read and enjoy a good fanfic based on this scenario -- but it does mean that I'm glad the show didn't go that way.
That said: I think that my feelings about any scenario ultimately depend on how it was done. As I said, when I first heard about the Season 2 finale I thought it was a destruction of the G/M ship and a victory (albeit a tragic one) for R/M. So, maybe the "Guy's noble sacrifice" scenario could have been done in a way that respected G/M, e.g., by showing that Marian does have very real feelings for Guy and regrets about what might have been, but ultimately chooses to follow her commitment to Robin. I suppose I could have lived with that if they did that on the show.
So... what do other people, particularly G/M shippers, think of this? I'd love to hear all opinions and preferences, but please, no matter what your 'ship ... no flames or name-calling.
Note: At first I wanted to link to this discussion only at the guyxmarian LJ community, but I'd also like input from others, including Guy fans who aren't necessarily Guy/Marian shippers. I do, however, have a request. If you have no sympathy for Guy, please stay out of this discussion. I really, really do not want to reply to posts along the lines of "Guy is a horrible man who should suffer and doesn't deserve love or redemption." If that's your opinion, you're entitled to it, but please express it somewhere else. Also, I do not take kindly to Marian-bashing, just to let you know. If you want to express the opinion that Guy deserved better than Marian, fine, but name-calling is unnecessary, and as far as I'm concerned any suggestion that Guy was "justified" in killing Marian or that she "deserved it" is way beyond the pale.