High-level ab initio calculations on silatropylium (1-Si) and silabenzyl (2-Si) cations and seven of their low-lying isomers (3-8), as well as on their carbon analogues, tropylium (1-C) and benzyl (2-C) cations, are reported. Heats of formation have been calculated at the G2(MP2) level of theory with the help of isodesmic and isogyric reactions. Relative stabilities and hydride affinities are reported using G2(MP2), G2(MP2,SVP), and density functional theory (B3-LYP, B-LYP, and B3-P86) procedures. The calculations confirm the experimental finding that tropylium cation is lower in energy than benzyl cation. The calculated heat of formation for benzyl cation (Delta H-f298 = 907 kJ mol(-1)) is in good agreement with a value derived from recent experimental data but the calculated heat of formation for tropylium cation (Delta H-f298 = 878 kJ mol(-1)) suggests that an experimental re-examination would be desirable. The stability ordering is reversed for the silicon analogues, silatropylium cation (1-Si, Delta H-f298 = 980 kJ mol(-1)) and silabenzyl cation (2-Si, Delta H-f298 = 942 kJ mol(-1)), with the latter lying lower in energy by 38 kJ mol(-1). Among the isomers that we have examined, the lowest in energy by a considerable margin is (eta(5)-methylcyclopentadienyl)silanium cation (8, Delta H-f298 = 839 kJ mol(-1)). Two other isomers, delta-silabenzyl cation (3, Delta H-f298 = 969 kJ mol(-1)) and (eta(5)-cyclohexadienyl)silanium cation (7, Delta H-f298 = 965 kJ mol(-1)), lie intermediate in energy between 2-Si and 1-Si. The implications of our theoretical findings with regard to recent experimental results on the relative stabilities and hydride affinities of the C6SiH7+ isomers in the gas-phase chemistry of silatoluene radical cation are discussed. Our calculated relative energies and hydride affinities suggest 8 as the most likely prospect for the second C6SiH7+ isomer (in addition to 2-Si) observed experimentally.