Faith is more than worthy of Spike. We see Spike fall in love three times. Each time with a woman that is totally out of his league. After Buffy, there really is nowhere left to go, though.It is an insult to say that Faith isn't as worthy for Spike, who is the cheater in this situation, as Buffy is.
The thing is, Spike's devotion to his chosen woman is his one redeeming feature. If you take that away, then he is just a regular douche bag.