Do you use Luster or Metallic for large prints?

I just sent off for a 16x24 metallic print and it will be framed. I was wondering what is the general rule with large prints using Luster or Metallic? I always thought anything that will be handled and touched (ex.4x6's or wallets) should be luster and everything larger should be metallic.

I have been pleased with large prints in metallic. I did have a problem with large metallic prints mounted on foam core with the grain of the foam being visible in the print.