The Mezzanine

There's some evidence that vampires get tougher and harder to kill with age (The Master, Kaikistos needing to be killed extra hard) but I don't think it's ever been stated definitively in canon that they get stronger. I don't think that it's an unreasonable extrapolation that they do, but I'd definitely class it as na extrapolation rather than hard canon.

OTOH I think it's also reasonable to assume that Angel has more brute strength than Spike simply because he's bigger and bulkier.