What would be the best option for sunken/hollow temples, filler or implants? (photo)

I think filler is a better treatment option. My preferred filler for the temples is Sculptra Aesthetic. Results can last up 2.5 years although should be maintained with yearly treatments. 

Thank you for your question and photo.
I do see the changes that you are noticing in your temples. In my experience, Sculptra is the best option for treatment. Sculptra is an injectable filler that has long-lasting results and is very effective for this particular area.
Consider seeing an ASOPRS oculoplastic surgeon in your area for a consultation.
Best of luck!