Well to be honest...
Xander , Anya, Tara and Dawn won't be useful in a fight. If we include Faith, I feel like she'd be far more loyal to Angel's team than Buffy's, she has a better relationship with the Fang Gang. Spike on the other hand has been apart of both teams, I feel like he would still go to Buffy's side.
Comparing Gunn and Wes to Xander, Anya, Tara and Dawn would be rather pointless. Since the two members of the Fang Gang are actually useful in a battle.
I think the match ups would go like this.
Buffy vs Angel - Angel's tough but Buffy could take him. Buffy's physical power << Angel's physical power, however, I do believe that Angel has a fighting chance since he's far more ruthless than she is and if he's trying to gank her, then, she could end in the fight of her life.
Spike vs Faith - Faith has more physical power, considering that Spike doesn't have as much raw strength as Angel. But at the same time, he's capable of soaking up disgusting amounts of damage, so I'm not sure if Faith will be able to kill him before he inevitably murders her. Regardless this could go either way.
Illyria vs Dark Willow - If Illyria was at full strength, she'd probably just manipulate time and murder her.
I can tell you this much, when it comes to the greater good. AI is willing to do things that Scoobies would never even consider, they're just as cut throat as Giles or Spike. If they had to sacrifice Dawn to save the Earth, I can't imagine all of them would be on board with it, but they'd probably go through with it.