What would the utility function of gold and silver be? Does it change depending on the role of such metals in money vs commodities?

I can't figure out if it would be in the form of u(x_1, x_2)= a(x_1)^alpha + b(x_2)^beta [substitutional] or something else. I know it's subjective to the individual, but I'm just looking for a general model. Another idea I have is u(x_1, x_2) = max{x_1, x_2} because I expect most people want more.

I realize the question is worded kind of in a weird way, but I hope the idea is clear.