Abstract: The long-term atmospheric evolution and ion escape of unmagnetized terrestrial planets under continuous solar wind erosion are fundamental to understanding their habitability, water loss mechanisms, and evolutionary pathways. Although Venus and Mars both lack a global intrinsic magnetic field, their ionospheric structures, surface magnetic field distributions, and atmospheric compositions differ significantly, leading to distinct types of induced magnetospheres and ion acceleration environments. Despite extensive observational and modeling efforts, the physical mechanisms governing the differences in their electromagnetic structures, ion dynamics, and escape fluxes remain insufficiently constrained. To address this issue, we develop and employ a unified three-dimensional multi-fluid magnetohydrodynamic (MHD) model to systematically compare the global plasma structures, electric field systems, heavy ion distributions, and escape characteristics of Venus and Mars under solar wind interaction, and further evaluate the performance of Mars four-species single-fluid, four-species multi-fluid, and ten-species multi-fluid models. Our results show that the ion escape processes of both planets are jointly regulated by the solar wind dynamic pressure, Hall electric field, and ionosphere–magnetosphere coupling. Owing to its dense ionosphere and strong photochemical processes, Venus exhibits a pronounced magnetic pileup region, elevated heavy-ion densities, and a nearly symmetric electromagnetic structure. In contrast, Mars is strongly influenced by spatially variable crustal magnetic fields and more prominent Hall effects, which produce complex ion-scale structures and acceleration pathways at the magnetic pileup boundary, dayside plume region, and magnetotail. The Hall electric field contributes a substantially larger fraction of the total electric field at Mars than at Venus, even dominating the local electric field direction in plume source regions and plasma sheet locations, indicating a stronger electromagnetic acceleration capability under comparable upstream conditions. Comparisons between simulations and observations show that the ten-species multi-fluid model most accurately reproduces the bow shock and magnetic pileup positions, ionospheric density profiles, local electric field strengths, and overall ion escape rate (~1.4 × 10²⁵ s⁻¹). The model also reveals a pronounced mass dependence in Martian ion escape: light ions (e.g., O⁺) are preferentially accelerated into the magnetotail and dominate tailward escape, whereas heavy ions (e.g., \mathrmO_2^+ , \mathrmCO_2^+ ) primarily escape through dayside plume structures. These patterns reflect the combined effects of multi-species coupling, Hall physics, and crustal magnetic field geometry. In summary, Hall electric fields, multi-ion coupling, and localized magnetic field structures are identified as key controlling factors that shape the electromagnetic configuration, ion transport pathways, and long-term atmospheric loss of unmagnetized terrestrial planets. The findings deepen our understanding of the physical divergence between Venus and Mars and provide new theoretical constraints for modeling atmospheric evolution and assessing planetary habitability.