I have a leak in galvanized supply pipe above kitchen ceiling (upstairs bathroom). This will be replaced... is copper or Pex better?

IPC does allow PEX in concealed spaces with stipulations as to the connections, your local code may or may not allow same. Personally, I would not use retrofitted PEX, with connections, in a concealed overhead space. Full line pass through with manifold fittings is OK. PEX is easier and cheaper but Copper sweat fittings are better. Just had a brand new house in our neighborhood pull a PEX fitting out, in the garage ceiling, with a lot of water damage. Do not use Sharkbite fittings in concealed spaces.