The performance of resonant sensors based on nanoelectromechanical systems depends critically on the maximum amplitude of oscillation reached in the linear regime. The maximum linear amplitude is determined by nonlinear mechanisms that can originate from the material, geometric and transduction mechanism related factors. Here we compare the two competing effects, the geometric and drive-induced nonlinearities, for a commonly used device family, the thermoelastically driven, doubly clamped beams. We find that the geometric nonlinearity dominates for most of the device designs used in the literature, however the drive-induced nonlinearity becomes the determining factor for thicker beams with small electrode lengths.