Tin Bar

Tin is the alternate roll for the first ore tier and it is slightly ahead of copper in most respects rather than being an even swap. If your world generated tin you came out marginally better. There is no reason to trade for the other one unless you want the specific look of the armor.

Your starting tools are always copper, whatever the world rolled. A fresh character spawns with the copper shortsword, pickaxe and axe even in a world where copper does not generate at all, so the gear in your hotbar tells you nothing about what is in the ground. Dig a hole and look at the ore before you plan a smelting run, and check the Copper Bar page if you are trying to work out which one your world has.