In terms of risk vs. reward I gotta go with SLTD (I know, shocking).
The risk with SLTD is mitigated because of the installation side. So even if the cell somehow doesn't work out, there is still very good odds of a great ROI at this point since the SPO dilution is now already baked in and what is left is just $25 mil to create shareholder value with. Only up from here.
And of course the upside is ridiculous if and when the cell hits.
Now the big boys of solar, SCTY AND VSLR, are given their high market caps based on their giant retained revenue accounts. But I do see an unforeseen risk with that 20 year lease retained revenue model in that the panels they've used and own being a solar utility will be 100% obsolete within 2-10 years IMO. And they will be left with billions of dollars worth of outdated panels.
So that retained revenue account may be overvalued IMO.