If you want to have 32-bit compatibility, stick with Slamd64. Bluewhite64 is updated far more often though, but it appears to diverge strongly from official Slackware (unlike Slamd64, which seems to stay a little closer to a direct 64-bit port -- while maintaining 32-bit compatibility). However, I have never tried either, so I could be completely off.
There are also political implications of using one or the other, if you care about that sort of thing. Search the forums for Bluewhite64 if you want information about that.