[WillStewart]

Utilizing Isengard, RPi2/OSMC, and TVHeadEnd, I have a Vizio TV with optic connectivity to a 5.1 receiver and have Stereo Upmix enabled in ActiveAE, which is supposed to give me 5.1 upmix from stereo. Is there any other benefit I will receive from Free Surround Sound?

[AchimTuran]

So far this really depends on your own preferences and listening requirements.

So far I know ActiveAE only copies the e.g. stereo content to a 5.1 speaker layout. It probably does some mixing with the center channel. FernetMenta or Fritsch could tell more about the technical details.
I guess it is very similiar to your AV-Receivers multi-channel stereo mode.

Consequently adsp.freesurround uses a different algorithm to upmix the channels and uses customized filter and frequency bands to upmix to a 5.1 speaker.

In view adsp.freesurround sounds much better than ActiveAE's upmixing algorithm. Currently it is a littlte bit buggy with latest master branch. Futhermore I also like it more than my AV-Receivers dolby pro logic II, dts neo ..., because the voice that is coming out from the channel sounds so much clearer than any other upmix algorithm I heared before. But this is are really my own opinion others might have a different one.