To be completely honest in this particular setting neither of these proteins will show any presence in the plant unless you're a whiz with protein signaling.
If you can get luciferin to be expressed in the right tissue, correctly fold, and then maybe be stored somewhere (some vacuole or empty lysozyme) you'll still run into the issue of having to feed it ATP to cause the glowing. And even then, it will glow constantly.
(Check out pyrosequencing if you want to get a detailed description of how luciferase works)
There is the possibility that you can control the expression via a gene signaling system like the Lac operon, but I'm not overly familiar with plant systems.
You could add the gene several times to make sure it's in the correct section of the genome (Agrobacterium are somewhat undirected in their insertion methods) and try to include a genetic signal that causes expression / is turned on only in the flowering tissue.
GFP is most commonly used as a protein conjugate in order to observe where the protein is located in the cell. So the choice is really going to depend on what you're using it for.
When you are trying to just make something glow for the sake of curiosity / wonder I would suggest trying to use as little ATP as possible. For the health of the plant.
I hope this helps.
-SG