DEM related question

Dear colleagues, I’d like to have your opinion on DEM usage for studying deformations at small areas by DInSAR. I have several options for DEM - the build-in in SNAP, an ASTER tile and local DEM. Several questions arise - if using local DEM covering only part of the SAR scene what the results should look like. If using ASTER tile shall the results be better than using SRTM 1arcsec. What about the areas where no DEM is available.

Thank you for your comments/suggestions.