The focus Sash always leaves you with 1 HP after a potential OHKO. But the Focus Band has a 10% chance of doing this, but it will still exist once used. With the Focus Sash, it disappears after you have endured the OHKO. But the Focus Band doesn't. Also, the Focus Band works while not on full HP, unlike the sash.
It still really doesn't make much of a difference, though. So the Focus Band still isn't very good, and isn't used very much.
I hope I helped!