For speaker LED rings, I've settled on 3D Solutech "clear" PLA. Once it's printed, it is less clear (more translucent) than the PETG or other PLA I've tried. White looked pretty crappy.
Every difference in print settings (infill, diagonals vs concentric, print orientation) will affect how it will transmit light. I would think solid would transmit more light. An air gap may do a better job of spreading the light due to different angles of refraction.
I'd start with picking the orientation to make it strong, then play with settings a bit. It's a small part, so not too much time or material wasted during tests. I'd be tempted to print it on it's back (as shown in your pic), but the mounting tabs could be prone to breaking.