Unsurprisingly I have a bias for Goldbacks, but it is rooted in some experiences in gifting and tipping.

A quarter gram or half gram of gold (see monarchpreciousmetals.com) is very unimpressive size-wise. I think it is hard for people to grasp the very tiny flake of metal has any value.

On the other hand, Goldbacks look like, and can be handled like, the cash they've used their whole life. I've noticed it seems to "click" with them and they seem pretty pumped to have spendable gold. Plus, they are works of art and nice to look at.