As many of you guys said, I think she would have ticked the option 'None of the above'. Both guys were vampires and so immediately made them inappropriate. I think she always saw Spike as less of a threat in the sense of relationships, even after the revelation in Crush, because that's what they all presumed it to be; a crush. "Some weird Spike thing". However, she has The Talk with Angel in The Prom because I think she knows that what's between Buffy and Angel is huge, is the big love and so he needed to leave before the two got in even deeper than they already were ("When it comes to you Angel, she's just like any other young woman in love. You're all she can see of tomorrorw"). Didn't The Watcher's Guide say something about just seeing him as a man who was wrong for her daughter? But with Spike she never so those feelings that he eventually developed ever reciprocated from Buffy so there was never the same concern. So I don't think it was ever a preference for Buffy, but more the one she deemed less of a threat in the 'Developing as a couple' department. When Spike became a slight threat in that matter in Crush she suddenly went from laughing with him over gallery order slip-ups to saying 'I don't like this, he could become dangerous...and Spike he's pretty [twisted]'. Fickle much?
As people though, I think she always preferred Spike because her experiences with him were less...I ant to say less dark but the first time she meets him he's trying to kill her daughter which isn't exactly Lollipop Land Light. But He never directly tried to kill her, or taunted her with the concept of sleeping with her daughter, etc etc. Actually in Lovers Walk she's fully under the (mis)understanding that Spike is the good one and Angel is (still) the bad one. Which is why when Buffy attacks Spike and invites Angel in I think you hear Joyce say 'Okay, I'm confused again'. But once she learns Angel's good she seems to deal with him quite well. I think it's Enemies when he and Faith turn up (Him pretending to be evil) and Joyce says hello to him quite pleasantly, even joking with him over hoping there's no vampire attack coming. Plus, she totally glows in a smug little get-me way when he compliments her hair. (The first time I saw that little bit, it was such an Angelus thing to do it gave me little mini chills). But I think her preference always leant more to Spike personality wise because she had more good experiences with him than she did with Angel. Plus Spike was good with Dawn. Admittedly, we never see Angel with lil sis but considering his broody nature we can presume they weren't braiding each other's hair.
Now, I've gone to a really amusing visual place...where was I?
Ah yes, boyfriend choiceage. I think if she had to pick any boyfriend Joyce would want Riley for Buffy if we discount the whole vampire whores thing from Into the Woods. Because up to that moment I think it's everything that Joyce wanted for Buffy; a normal guy, a good guy, family guy (not the TV show though...), and knew about her Slayerness and accpeted it so Buffy had nothing to hide, plus he was the strong protective guy at the same time. Could he be a more perfect potential son-in-law?
I think Kristine Sutherland says something about this choice in an interview though...let me find it. (I realize since I'll post this as one none of you will even notice the time it took to find the interview). Ah here we go, she was asked 'As Buffy's Mom, who do you think is better for Buffy?'. She replied:
"Oh gosh, as Buffy's Mom...Riley. But as an audience member and great romantic, I love that, that relationship between Buffy and Angel. But as Buffy's Mom I would have to say that Riley is probably the healthier choice!"