There is a difference between the two materials, but it doesn't show up very well in the photos. With a 1.0mm thickness, the S&F looked "grainy" - similar to Photoshop's add-gaussian-noise filter. The PLA was striated or slightly striped. Neither is noticeable from more than a few inches away however. A quick sanding on the PLA print's top layer would probably eliminate the stranding.
The PLA did seem to reduce the amount of light a little more, but it diffused the light more. Neither material significantly affected the colors. I probably wouldn't trust either to set my white balance, but for a rainbow light show with NeoPixels both materials work great.
|Here's the case design. It uses a first generation Trinket from Adafruit, a 12 pixel ring and a potentiometer to change effects.|
|A pic with the cover on the case. This uses the Strong-and-Flexible material from Shapeways.|
A close-up of the S&F material - notice the grain.
Overall, I'm very happy with the cheap PLA as a diffuser material. I will play around with other brands of PLA soon, and I understand some are much more translucent than the basic E-sun.