Of course at the end day this is largely about subjective preference. No character is perfect and without flaws and neither of these relationships are written to have no reasonable points for criticism, or to be without any foundation for praise either. Both Angel's and Spike's stories are written to be about complex individuals who are trying to walk heroic paths. We just join them at very different stages and watch different aspects of their stories as they progress. They are individual characters with their differing points of focus, their own strengths and their own weaknesses. When souled I'd say they are both capable of having a healthy, adult relationship and Buffy has actively/openly chosen to pursue a relationship with them both at different points. In my opinion she clearly loves them both, cares for them both deeply and they have both been written as legitimate relationship choices for her when they're souled. Of course writing intention doesn't mean that it has to work for you and obviously on both sides of this shipping war it doesn't always compute for some viewers/readers. And even if they both do make sense to you, people will still often have a personal preferences. I think it is fair to say though that to see either souled vamp as a terrible person or either relationship as totally invalid and/or beneath Buffy, is to miss intended qualities of the characters or aspects of the relationship or the progress in the story within their dynamic together. Things written that others clearly do see and appreciate. Sure they are characters that have flaws but these stories are about characters who are trying to walk positive paths, how they fail but also how they succeed. Both relationships are written as having strengths, both vamps as characters that Buffy cares for, chooses to be with and both as relationships that have brought her both heartache and strength.I don't understand how one could compare Spike with Angel as the best man (vampire) for Buffy and choose Spike? As you say they had a horribly destructive relationship and it blows my mind that there is such an overwhelming response to this ship at all. I find Spike beneath Buffy. Angel to me is her equal.
Both vamps are favourite characters for me but I prefer Spike and I think that he and Buffy strengthen each other and complement each other very well. I totally accept some people prefer Angel and would prefer Bangel, but I don't want to get into listing any weaknesses that I feel exist in their relationship to raise up Spuffy. As I've said neither are perfect or without points for criticism or praise. Unfortunately for Buffy and Angel their situation isn't without additional complications and the potential risks of them being together have factored into keeping them apart. Buffy and Spike are lucky that they can have a shot at being together and are, at the moment, happy and it is working for them. I'll enjoy it whilst it lasts and hope that if it does fail that they do both gain from the time they've shared. Pockets of relative peace and happiness are often few and far between in this verse.