We have used high-precision, high-resolution digital terrain models (DTMs) of the NASA Mars Science Laboratory (MSL) and Mars 2020 rover landing sites based on mosaicked images from the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (MRO HiRISE) camera as a reference data set to evaluate DTMs based on Mars Express High Resolution Stereo Camera (MEX HRSC) images. The Next Generation Automatic Terrain Extraction (NGATE) matcher in the SOCET SET/GXP † commercial photogrammetric system produces DTMs with relatively good (small) horizontal resolution but high error, and results are terrain dependent, with poorer resolution and smaller errors on smoother surfaces. Multiple approaches to smoothing the NGATE DTMs give very similar tradeoffs between resolution and error. Smoothing the NGATE DTMs with a single pass of an area-based matcher, which has been the standard approach to generating planetary DTMs at the U.S. Geological Survey (USGS) to date is probably near-optimal in terms of both combined resolution-error performance and local slope estimation, but smoothing with a 5x5 lowpass filter performs as well or better. DTMs from the HRSC team processing pipeline fall within this same trade space but are less sensitive to terrain roughness. DTMs produced with the Ames Stereo Pipeline also fall in this space at resolutions intermediate between NGATE and the team pipeline. Although DTM resolution and error each vary by a factor of two, their product is much more consistent, varying by <20% across multiple image sets and matching algorithms. Refinement of the stereo DTM by photoclinometry can yield significant quantitative improvement in resolution and some improvement in error (improving their product by as much as a factor of two), provided that albedo variations over distances smaller than the stereo DTM resolution are not too severe.