Metal cuffs are very uncomfortable and are likely to cut off circulation, in my experience. Leather is much better as it can, with use, conform to the wearer and is less likely to cause chaffing.

If you're going to be wearing them for any length of time, leather's just generally a lot more comfortable than metal.

I prefer leather. To me, metal cuffs should be reserved for serious S&M.

I prefer leather for everyday use, however, I would like to buy a good pair of heavy duty shackles.

soft cuffs. metals ones can do damage.

I much prefer leather or neoprene cuffs. Metal "Police Style" cuffs can cause constriction damage and even nerve damage if not used carefully. They leave marks where I'd rather not have them. I find them distracting and very uncomfortable, although they look good for photo shoots and things like that. For real hard play, nothing beats safer soft material cuffs.

Leather! Metal may be shiny, but leather goes the distance.