There are two known competitive arrangements for reflection-type coaxial holographic storage systems: the optical layout developed by OPTWARE, which is a so-called collinear or ''split-aperture system'', and the ''common-aperture system'' developed by the European consortium named ATHOS. We modeled the reference diffraction noise for both arrangements by the beam propagation method (BPM), and compared the results from the M number (M#) consumption point of view. We suggest the use of reference beam apodization for the split aperture system to reduce the diffraction noise caused by the reference beam at the object area.