I used both on a couple of different sites before I launched Linuxlaboratory. When I was figuring out what to do there, I settled on phpBB. Newer versions of it have a few more bells and whistles, and YABB probably has even more now (as evidenced on this site), but if you just want the basics, and some emoticons and other stuff, phpBB is very good. Very solid and stable, though so is YABB, now that I think about it. I guess it really is personal taste. I think what got me using phpBB is that it's written in PHP, which I'm actually more comfortable with than Perl, which is what YABB is written in. The rest of my site is all PHP, so it was a consistency in maintaining the site that I liked.