Contemporary x-ray radiotherapy employs small radiation fields to deliver highly conformal dose distributions. Submillimeter accuracy in the measurement of the delivered dose map is a crucial requirement of detectors proposed for quality assurance applications. 2D monolithic silicon array detectors can provide high spatial-resolution by optimizing small sensitive volumes (SVs) in a large active area. They offer a stable and near energy-independent response in megavoltage photon beams, good dose linearity and real-time read-out. The SVs are ion-implanted on a silicon wafer whose geometry and physical characteristics, such as resistivity and defects concentration, dramatically affect the detector performance. The Octa is a novel 2D monolithic silicon array detector dedicated to small-field dosimetry. Its 512 diode-SVs are arranged with a sub-millimeter pitch along 4 intersecting orthogonal linear arrays. We report on the experimental and numerical characterization (performed with Sentaurus Workbench within the Synopsys framework) of two Octa detectors, manufactured respectively on a bulk and on an epitaxial silicon substrate. The effects of resistivity and defects concentration profiles across their large-area monolithic silicon wafers is compared and discussed in terms of the response linearity with dose, response uniformity, charge-collection efficiency and clinical performance in the case of a small radiation field delivered with a flattening filter free beam.