I'd consider Edward outnumbered really, since Bella (even as a vamp) would be totally useless in a fight.
Buffy could take them both out alone, but with Spike as back-up then I definitely see them winning.
On top of that, they would just work much better in a fighting team than Edward and Bella, because Buffy and Spike respect eachother. They are not overprotective they have trust and respect for the strength and skill of the other, so could effectively fight. Edward has no respect for Bella in that way, he sees her as weak, he would be completely distracted in a fight if she was involved because he doesn't think she can fend for herself. He'd be trying to hide her behind a tree or something, while Buffy and Spike were busy being kick ass :)

Buffy is the fucking slayer for good sake! she brought down THE FIRST! she would have taken them alone! but whit spike laughing off them. edward would just sit down and cry

Totally agree wit CupcakeKiller ;D

the fight would actually be more like buffy and spike vs edward cos even if edward did let bella fight (which btw is very unlikely) she couldn't really do anything..maybe she'd just pick up a stick and start hurting herself as a distraction or something but no one would really notice cos buffy and spike would be too busy totally kicking edward cullens ass.....i think spike could even take on edward alone...

Spike WOULD kick his ass alone, and agree about the bella bit ^^
and agree with cupcakekiller ^^

edward would be too concerned with protecting bella...spike and buffy will fight for their own hides...

edward and bella would so lose they're wimps Buffy and Spike are awesome!

They wouldn't stand a chance.