Both linear and nonlinear sliding surfaces are appropriate to the sliding mode controller or even variable structure controller. The selection sliding surfaces can be from quadratic functions or even energy dissipation functions as long as these function can be evaluated by Lyapunov stability theory. The constants (as I understood, it might be the coefficients of sliding surface functions) selection must take care of the transient response of the whole system if you linearized your system around the equilibrium (or equilibra)