The whole Angel/Shanshu/Redemption/Mortality thing never made any sense to me anyway.Angel, as was pointed out, had mortality and gave it back. His reason was that he could no longer be an effective warrior for the Light (oh, and by the way, he can't "protect" Buffy anymore, and he might even get her killed by HER trying to protect HIM) which is a crock, because we have seen plenty of completely normal humans who were quite effective demon fighters, beginning with Giles and Xander, then Riley, even after his chip/drug enhancement was removed. The other members of Angel's own team were human, and they did quite well in the vamp dusting department, even Fred. Wes compensated for his lack of physical strength by using a variety of weapons, up to and including firearms. Gunn even had his own crew of demon fighters (who, toward the end were also armed with modern weapons). Angel could have been almost as effective a demon fighter as a human as he was as a vampire. The real reason for his refusingrtality might be something else entirely.
In Los Angeles, as a vampire demon fighter, Angel had no equal. He was the champion, the Black Knight, helping the helpless. Without his vampire powers, he would be, at best, just one of several fighters. If he remained the best, it would not be by nearly the margin that he was as a vampire. Quite likely, there was an ego issue involved that he was either not conscious of or was unwilling to own up to. Despite the reasons that he gave, that might have also been a factor in his leaving Sunnydale in the first place. With Buffy there, he wasn't the alpha fighter. As Angelus, he relished the domination of his victims, avoiding real confrontations unless they were forced upon him. He was, in the most basic terms, a bully. He was afraid of and ran from Holtz, who was, granted, a skilled fighter, but still only a mortal. Angel, by his very reasons for refusing mortality in the first place, was basically no less a bully than Angelus was. They are, after all, when all is said and done, the same person.
Then the Shanshu Prophecy is introduced into the storyline, and we are supposed to believe that this is going to be Angel's Holy Grail, the reward he will receive for his years of good deeds ("It's really the redemption," he would say, "The mortality is just icing on the cake."). We are supposed to believe that suddenly Angel is not only willing to take, but is actively seeking, his mortality, including his loss of power AND his mortality. Yeah, right.
I honestly believe that Spike, on the other hand, only wants the Shanshu because Angel wants it. He doesn't strike me as the type who would give up his immortality willingly, nor do I believe he would, if he received it, consider it a reward. As much as he loves Buffy, I can't see him actually wanting to become human just to be with her. (On the other hand, that just might have been a deciding factor in why he didn't go to Europe to find her when he became corporeal. Maybe he stayed in LA on the hopes of becoming the man she deserved, even to the point of seeking out his mortality the same way he sought out his soul. I'd never thought about that until just now. That might be something to explore in another thread). On the other hand, Spike hates Angel, and has been dominated by him for decades, and taking the Shanshu away from him would suit him just fine.