Adsorption is an important process used in many industrial unit operations. However, due to the heterogeneity of surfaces studied experimentally, it is not a simple task to evaluate the validity of theories for adsorption. Here, we present a thorough investigation of density functional theory and density gradient theory based on molecular simulation. While comparisons between these theories have been made in the literature, only our recently developed equation of state (EOS), the Perturbed Truncated and Shifted (PeTS) equation of state and the according functional version, allow a thorough validation of both theories, because the EOS separates repulsive and attractive free energy contributions consistently with the inhomogeneous theory. The PeTS EOS represents the thermodynamics of the Lennard-Jones truncated and shifted fluid with a cutoff radius of 2.5 times the fluid diameter very accurately and is valid in the metastable range, too. To check the validity of both density gradient and density functional theory in adsorption scenarios, molecular dynamics simulations are performed for several wall potentials common in adsorption calculations at various states and solid-fluid interaction energies. A new parametrization for the density functional theory is proposed that relies on bulk data for the pressure only. Adsorption is predicted well from this new theory in both gaseous and liquid states. In contrast, density gradient theory turns out to be valid only in the dilute gaseous regime if the parameters are not fitted to adsorption.