If it is your first pair: fur lined, not metal. Comfort is what decides whether there is a second session, and metal is the most common reason there is not.
If wrists sit outside a standard range: buckles over hook and loop. Buckle straps adjust across a far wider span, and fixed sizing is why a lot of restraint gear never gets used twice.
If they are going to stay on for a while: check for two fingers of room. You should be able to slide two fingers under the cuff. Tighter than that restricts circulation rather than movement.
If you live with other people: silicone or fabric. Chain and buckles are considerably louder than anybody plans for.
On safety: keep the release within reach, every single time. Key in hand for metal, scissors nearby for anything tied. Check hands for cold, tingling or color change rather than waiting to be told, because the person cuffed often will not notice until it matters.
