We present a simulation study of silicon nanowire transistors, based on an in-house code providing the self-consistent solution of Poisson, Schrodinger, and continuity equations on a generic three-dimensional domain. The main assumption, based on the very small nanowire cross section considered, is that an adiabatic approximation can be applied to the Schrodinger equation, so that transport occurs along one-dimensional sub- bands. Different subband transport models are considered, such as ballistic transport, either including quantum tunneling or not, and drift-diffusion. We show that nanowire transistors exhibit good control of short channel effects, and that barrier tunneling is significant in the strong inversion regime even for longer devices, while it is significant in subthreshold only for the shortest channel lengths. Finally, we show that a subband-based transport model allows to reach a very good trade off between physical accuracy of the simulation and computing time.