Decide day or scene first. Everything else follows from it. If it needs to survive a work day and a coffee run, you want thin, soft, and no obvious hardware. If it only comes out behind a closed door, buy the leather one.
Check the ring before the look. A collar with no D-ring or O-ring cannot take a leash, and a lot of pretty collars have none. If a lead is ever going to be part of it, confirm the ring is there and that it is welded rather than a split ring that can pull open.
Sizing: measure your neck, then add two fingers. Every collar here is adjustable, but adjustable has a range. You want to land in the middle of that range, not at the last hole. Two fingers should slide under it comfortably. If they do not, it is too tight, and this is not a category where you tough it out.
Locking collars are symbolic, not secure. A heart lock is a gesture. Do not treat any collar as restraint, do not sleep in one, and keep the key somewhere both of you can reach it.
Material: leather ages, satin does not. Real leather softens and takes on the shape of the wearer, which is part of the appeal. Satin and faux suede stay as they are, wash more easily, and sit better against sensitive skin. Neither is the better choice, they are just different commitments.