Well Spike isn't the champion. Lyndsay just told him he was to mess with his head and help further mess with Angel. Had Spike attacked Angel, she would definitely have taken Angels side at that moment in time......as she was kissing him I imagine....that tends to show ones allegiance I feel....
Actually, Buffy named Spike as her Champion well before he ever met Lindsey. Granted, it wasn't until after she sent Angel away that she dd that, but I think that she saw him that way well before that particular episode. And, as for whose side she would take, she had already taken Spike's side against the objections and/or disapproval of everyone in her household except maybe Faith and Andrew, and had gone so far as to close the door in Giles' face when he tried to make excuses for what he and Wood had tried to do and failed.
I'm not trying to start up the old "Does Buffy love Angel or Spike the most here," I'm simply stating that Spike was a member of Buffy's team, was, by her own assessment, "the only one watching her back," and, as had been shown several times during that season, she considered him under her protection to the point that she took his side against all of the objections of all of the people who she should have trusted and depended on the most. In this instance, under those circumstances, I can't see her siding with Angel any more than Angel sided with her against Faith. It has nothing whatsoever to do with who loved who the most, who was kissing who, who just popped in for a quickie, or who had done the unthinkable by actually fighting for and winning his soul back.
At that time, Spike was a part of Buffy's team, and Angel was not. That says it all. If a fight had started between Angel and Spike, regardless of who started it, she would have broken it up, without actually siding with either man, and she would still have done exactly what she did, which is send Angel back to LA where he had come from, out of simple logic. She needed troops who would fight the First Evil, not each other.
Once again, I AM NOT DEBATING WHETHER BUFFY LOVED ANGEL OR SPIKE THE MOST. Spike was a member of Buffy's team, and her first responsibility, as the leader, is to her team, not her boyfriend, whichever vampire that might have been. Spike stood up for her against everyone in Buffy's house when they betrayed her, no questions asked, and no hesitation, and gave her courage, support, and comfort during her darkest hours. I can't see her repaying his loyalty by doing anything less for him.