I got both this Everyday Layers Spin Slip and the Everyday Layers Full Slip. Posting this review in both products for folks who might be uncertain about which to get.
The Spin Slip feels a bit more substantial, which is great when you need it, but the straps also have more substance, which show a slight bump on your shoulders if it's under a light (for example, silk) or tight-fitting dress. For smooth shoulders under lightweight material, I recommend the Full Slip version. Now that I have them, I can see the difference in the photos here, too.
At the time I ordered, the SpinSlip was available only in the 18" length. FWIW that length is identical to the Full Slip standard length. I hope the longer lengths become available again.