Cocoa or Carbon?
This question is the stuff of flame wars. The merits of each are tricky and subjective. However as of Leopard, Cocoa got support for 64-bit GUI apps whereas Carbon support was explicitly dropped by Apple. They are both fully supported in 32-bit land, but the writing seems to be on the wall that Carbon is not quite as loved as it once was. Unless you have a legacy Carbon codebase to deal with, you should probably go for Cocoa.