Kuchiki wins for reasons already stated...
in essence, this fight is basically SF with shikai/shunko, vs Byakuya with bankai...
if Byakuya's bankai, which allows him to attack and defend at the same time, can keep up with bankai Ichigo's speed, I don't see why it would have problems against SF...
if SF is fast enough to blitz Byakuya before he goes bankai then she wins, I don't think she can though IMO...