This is subjective, naturally, but to be someone’s partner, they need to be your #1….it’s rooted in support, placing their needs before yours, always having their back, being their best friend and ally, loyalty, and lots of other things I’m sure I’m not eloquent enough to express. Fits Spike’s role for Buffy/in her life in S7 to a tee, and in my opinion, couldn’t be any further from teen-romancy stuff.
Joss' comment merely reflects how it's a mature/grown up relationship in S7, to counter the teenage thing.
Angel is capable of being these things for Buffy, but it will always come second to his redemption journey (IWRY, for example). And that’s cool. But that doesn’t make for a partner she deserves. She deserves to be someone's #1. Spike doesn’t have the same type of motivations/need for redemption as Angel which would supersede what he could be for Buffy IMO.