Which better? to be specialist in one field, or to get parts form each field so you will be aware of many fields? and why?

It depends on which field you deal with and which county you live, if you live in Arabic country and work in engineering you have to be a superhero and know everything in all fields...

Better become a specialist in one field rather than knowing partly in many fields because proclaiming you are a specialist in one field with only partial knowledge of it is dangerous.