While vanadyl mimics are frequently employed experimentally in place of the fleeting ferryl intermediate in C-H activation, we showed through an extensive computational study using DFT that there are crucial differences between ferryl and vanadyl intermediates in hydroxylase active site models that could be rationalized in terms of differences in their ground spin states. Our study of M(IV)=O isomers revealed not only that the most favorable metal-oxo isomers differ between Fe and V, but also that conversion between monodentate and bidentate isomers is energetically favorable for Fe but strongly unfavorable in vanadyl mimics.