Spike would win this. Especially if you are counting feats from the novels and comics. Spike has defeated gods. And as he said in the series and Buffy admitted, he and she only ever "danced". He didn't seriously put in the effort to really kill her, she was to much fun to play with. The Slayers he meant to kill, died.
Spike was never a typical vampire. He sought combat and trained. He seriously practiced martial art skills for over a hundred years. He also lead a vampire battle campaign that wiped out a large part of Europe at one point. It wasn't shown in the series but Giles mentioned it when they first looked Spike up in The Watcher's Chronicles. He wouldn't hypnotize helpless victims. He lives for combat. Consideres it great fun. "I'm in it for the rush, the crunch, the thrill." Even when you knock him down he just keeps coming. Spike kicks serious ass even when he isn't in full vampire fight mode. Once he let the demon lose, it would be all over.
I didn't make this video, but it does a good job of showing what I'm talking about. Unless Nightwing can turn into a pretty blonde girl and win Spike's love, Spike wins.