Light user data is the way to go if you literally just want to pass a pointer around. That's all a light user data is: a void* that is passed around in Lua values which can't be manipulated or modified by Lua itself.

A problem with light user data, however, is that you can't tell what _kind_ of pointer it is. It really is just a void*. So a C++ function bound to Lua could have _any_ light user data passed to it and you have no way to distinguish whether it's a GameObject* or a Texture*, unless you build your own system outside of Lua to identify these objects from a void*.

Full user data lets you attach a unique metatable to values which can be used to distinguish the types of the pointers they wrap. When you retrieve the void* encapsulated by the user data you can also grab the metatable and make sure it matches the one you expect so that you can tell those game objects and textures apart. Full user data is heavier-weight than "light" user data (obviously). There was a patch posted recently to the Lua user list that added a way to type-tag light user data without the overhead of full user data, if you really need the extra performance (you very well might not).