Couple of things - I'm not sure how much of the "Buffy is much stronger than Spike" is canon, and how much fanon. Although, I think she does say it somewhere... I don't know that it jarred me one way or the other, but I would probably have said it was a Slayer thing. She is stronger than Spike, probably, but not that much. Of course, Spike's strength and fighting abilities tend to come and go based on the needs of that particular episode... Consistency, thy name is not Whedon! Anyway, I think the having her be able to wield the hammer when he couldn't pick it up was just meant for a humorous bit and not to be taken overly seriously. That's how I saw it.