We study the effect of surface roughness (SR) at the Si/SiO2 interfaces on transport properties of quasi 1-D and 2-D silicon nanodevices by comparing the electrical performances of nanowire (NW) and double-gate (DG) field-effect transistors. We address a full-quantum analysis based on the 3-D self-consistent solution of the Poisson-Schrödinger equation within the coupled mode-space nonequilibrium Green function (NEGF) formalism. The influence of SR scattering is also compared with phonon (PH) scattering addressed in the self-consistent Born approximation. We analyze transfer characteristics, current spectra, density of states, and low-field mobility of devices with different lateral size, showing that the dimensionality of the quasi 1-D and 2-D structures induces significant differences only for thin silicon thicknesses. Thin NWs are found more sensitive to the SR-induced variability of the threshold voltage with respect to the DG planar transistors.