you don’t need to open new topics on any question, many of yours are quite related 
@marpet could you pleas join this with the other two:
I would suggest that you try “phase to elevation” instead and use the ASTER DEM as reference so both have the same base.
If both are in one product you can also create a scatter plot and see if the shift between both rasters is constant or just affecting values within a certain range.