I'm not sure I should answer since I'm not into men. I think if I were that it wouldn't make a big difference, especially as neither one would be my type (unless I was raised on different media that changed what I wanted). That said, at least Spike has a rare presence to him, for better and worse (and what most lead actors would want), and that doesn't change.
I like the earlier versions because they seemed to fit the story more. I have to overlook their age and pretend they still look the same as they get older. Because they're vampires, and they were turned when young, though they both, in their own unique way, come off much like the adults who won't leave home.
For Liam, he was too irresponsible (and probably insecure due to having a berating father too much like Wesley's, but his teen sister implies that he's probably not older than 20), and William was too much of an idealistic romantic who both sought the protection of his mother but also took as good care of her as he could (they did a good job of making him look younger for that flashback, and given his romanticism along with company, even if the runt, it suggests he was about 20 himself, possibly a bit older than Liam, before turned).
If the question is more about aesthetics and if I prefer younger and passionate to older and stable, I'd go with older. I think Giles would be my type, though there's plenty about Xander as well (which I'll explain if anyone asks) if I were young and foolishly searching for true love to give meaning to my life (though I don't think Xander is a bad catch and that we'd be compatible).
I've always been into older women (and the rare male crush also tended to be older, like when I had a crush on both Scully AND Mulder when I was 17-8), so I guess it's like a fine wine...but mainly I think I'm attracted to someone who has it together, some maturity (silliness or even playing with toys like Wash okay), and much more interesting to talk to. That they usually have less romantic and sexual hang ups (that afflicts both genders, if often in different ways) is a bonus as well. So I'm sure if I had a crush on Angel or Spike that I'd prefer them older for that.
But story wise, I prefer them looking younger.