Sexy and all that Spuffy were, they weren't built to last. .
However, I've kind of come to the conclusion that Buffy isn't "destined" to be with anyone,or in any way that's not what the show is about. She simply works best when she's on her own. If it was up to her though, and there were no curses or other implications to consider, she would definitely pick Angel over Spike.
If we get right down to the cold, hard truth, considering her calling, Buffy, herself, wasn't "built to last." The way that the whole Slayer dynamic was set up and maintained by the Shadow Men and Watchers, the Slayers were pretty much a disposable resource, easily replaceable when the time came. Remember, no Slayer in history has ever lived past age 25. granted, Buffy's chances are somewhat improved now, what with all of the other fresh young girls on the job now, but the fact remains that she is at the very best at very high risk now, where before she was at extremely high risk.
It's doubtful, despite what Buffy told Angel, she ever seriously allows herself to think "that far ahead," or she would end up more broody than Angel, himself. The best that Buffy can hope for, and I have no doubt that she is fully aware of the fact, is a few quiet moments when there will be someone there to "just hold her," or to let her share his bed, Angel breath or not. That doesn't HAVE to be Spike, but one thing is certain. Right now, it sure ain't Angel.
And right now is all that Buffy can be sure that she has, or ever will have.
If Buffy wants to play snuggle puppy with whoever is available, who, at the moment, happens to be Spike, then I for one am happy that she at least has someone to snuggle with. As for Buffy and Angel being "destined" to be together, well, so were Romeo and Juliette, Marc Antony and Cleopatra, Clyde Barrow and Bonnie Parker. Destiny isn't any guarantee of happiness.