There are certainly some parallels, and I wonder if the SPN writers were thinking of Buffy S6 when they wrote Swan Song. If so, they left out all the parts that made Grave actually good.
One parallel that particularly amuses me is the fact that on both shows, characters mock the idea that love might save the day. Willow says scornfully, mere minutes before she collapses in tears, "Is this the master plan? You're going to stop me by telling me you love me?" And I do find the following exchange between Dean and Adam, a few episodes before Swan Song, hilarious:
Dean: We're working on "the power of love.”
Adam: How's the going?
Dean: Mmm. Not good.
It's Jensen Ackles' delivery. Priceless.
But nonetheless, Swan Song does not manage to make me feel anything. So why does Grave work while Swan Song fails?
Part of it is this simple fact: In S6, Willow is the villain. She is influenced by magic, but at the end of the day, she is Willow. Xander tells Willow he loves her, in order to change Willow's mind about her actions. By contrast, Sam is not the bad guy of Swan Song. He's possessed by Lucifer. The message seems to be that Dean's love helped Sam gain the strength to overcome the possession, but that flies in the face of seasons past and seasons to be in terms of how possession works. Sam and Dean in later seasons are incredibly casual about torturing and killing victims of possession. If love can overcome possession, what does it say about all the people who can’t? Do they…just not love enough? All those strangers that the Winchesters don’t give a damn about? Do they not care enough?
In the past, some possession victims were able to overcome possession: John Winchester, Bobby Singer, Julia Wright from I Believe the Children Are Our Future. However, it was never portrayed as simply a matter of love.
(Sidenote: I read a tiny fic where Sam, in fact didn’t take control from Lucifer. What actually happened was Lucifer, upon experiencing Sam’s memories of Sam and Dean, relinquished control. Cool, but sadly not canon.)
And many other possession victims are not able to regain control. Sam in Born Under a Bad Sign, for one. Meg Masters, whose relationship with her sister is explicitly compared to Sam and Dean in Are You There, God? It’s Me, Dean Winchester, can't do it. Neither can Lisa Braeden, even when the demon possessing her holds a knife to her young son's neck. Ruby, Alastair, Castiel, Zachariah, Uriel, Lilith…if their vessels had just loved enough they would have been free? This is not a one-shot episode being undermined by Sam being able to overcome Lucifer through love. It's not lesser part of the mythology. This is something essential.
Another reason Grave works is because of the history of Willow and Xander's relationship. Xander, who has loved Willow as a friend since they were young children, is uniquely positioned to remind Willow that she was lovable even before she gained confidence and cool powers (something Tara could not do, as Tara did not know geeky Willow). I don't think Xander had ever declared his love to Willow directly to her face before, so Willow was hearing something new, even if she already knew it in her heart.
By contrast, Dean telling Sam that it will be okay is far from new. It's cliché. Dean's been saying that over the course of the entire show. Sam isn't being reminded of the importance of a relationship he forgot about: Even at the height of their problems in S4, Sam valued the relationship, and never forgot his debt to Dean. In The Monster at the End of the This Book, Sam tells Chuck, "Well, [Dean]'s looked out for me my whole life. I can't return the favor?" His relationship with Dean has been explicitly his driving factor all along. A few episodes before Swan Song, Point of No Return, in a much better way, demonstrated the power of the relationship and Sam's belief in it. "Sam, it's okay. It's okay. I'm here. I'm here. I'm not gonna leave you." is not a revelation on any level.
Now, if Dean had expresses unambiguous belief in Sam, that might be a little different. But, as in Sacrifice, Dean does nothing of the sort.
It's especially ironic, as over the course of the past couple of seasons, ever since Dean's Hell deal if not earlier, the show has been calling into question the idea that Dean's constant reassurances of Sam are good. This is in no way acknowledged by Swan Song. It feels, instead, like fan service - which is par for course in SPN, but still disappointing. Seasons 1-4 told a very complicated story that completely unique to these characters and their relationship, that led to their triggering the apocalypse. I would love to see a very complicated story completely unique to these characters that led to their love saving the world…but S5 wasn’t it. Instead, Swan Song felt like ending a rich banquet with a Jell-O salad.