I guess there was a misunderstanding: During the Range Doppler Terrain Correction, a DEM is required to estimate the geometric distortion of the radar signal according to the local topography. You are using the SRTM 3Sec which has a spatial resolution of 90 meters. As stated, this is automatically downloaded by SNAP, it exists since the year 2000.
However, this SRTM has nothing to do with your InSAR result. If you want to create a DEM from radar interferometry, you have to unwrap your interferogram and convert it to elevation. Have a look here: The Order of DEM Creating Steps
Unwrapping is the most important step, so you cannot skip it for DEM generation.
Besides that, I don’t expect a significant difference between phase of VV and VH.
The reason why both of your results are equal is that both simply show the SRTM digital elevation model at resampled spatial resolution of 14 meters (without any increase in quality).
The checkbox to include the DEM in the Range Doppler Terrain Correction module simply outputs the DEM used to estimate this geometric distortion, but not the elevation information from the interferometric phase.
Besides that, I have concerns that InSAR will work over your study area because you have lots of vegetation which causes massive temporal decorrelation within the 12 days between both acquisitions.
Have a look at your interferogram and the coherence layer. They indicate the quality that can be expected from the image pair.
Besides that, I don’t expect a significant difference between VV and VH, because the phase is not related to the backscatter intensity and should mainly respond the same. You can also check by comparing the interferograms. They should largely look the same.