Luciferase promoter vector over p-AcGFP1-C1 vector
You are confusing luminescence and fluorescence. GFP does not emit light. The abbreviation stands for Green Fluorescent Protein. You need to shine a blue light source at it and it will fluoresce green. The problem is that many organisms have background fluorescence, a cell without any GFP can often have a little bit of fluorescence by itself or contain pigments that mask the signal.  

Luciferase assays depend on a chemical reaction which oxidises a molecule (luciferin in the case of firefly luciferase) and photons are emitted.  This is much more sensitive than fluorescence but is also a lot more expensive and usually (but not always) requires cell lysis. It is very rare for a cell to emit light by itself so the background will be a lot lower allowing you to see smaller difference.

There are many reasons to choose one over the other in an assay, if you share a bit more on how you intend to use the reporter you can get a more detailed answer.