Often I just experiment with one or the other until I get the look I want and, they seemed to have the same effect on my image.
I finally decided to delve into the differences to try and understand them more.
This is what I learned.
--> The difference is that opacity affects all of the layer including layer effects.
Fill only affects the layer content, but not layer effects.
Here is an example of the difference.
The photo below has some text of which I used a drop shadow and a bevel. I didn't adjust either the opacity or fill.
The photo below I changed the opacity to around 50%. Everything on the text layer becomes 50% transparent including the text, shadow and the bevel.
The photo below I put the opacity back and changed the fill to 50%. The text started to become transparent but the shadow and bevel were NOT effected.
Interesting....With this in mind, I liked the look of the image coming through the text.
So, in my final image, I changed the fill to 0%. I then got the really cool effect of the text and photo below.
Wow, who knew that the changing the fill slider could create such a KEWL effect.