We apply the Levenberg-Marquardt minimization algorithm to seismic and classical observables of the alpha Cen binary system in order to derive the fundamental parameters of alpha CenA+B, and to analyze the dependence of these parameters on the chosen observables, on their uncertainty, and on the physics used in stellar modelling. We show that while the fundamental stellar parameters do not depend on the treatment of convection adopted (Mixing Length Theory - MLT - or "Full Spectrum of Turbulence" - FST), the age of the system depends on the inclusion of gravitational settling, and is deeply biased by the small frequency separation of component B. We try to answer the question of the universality of the mixing length parameter, and we find a statistically reliable dependence of the alpha -parameter on the HR diagram location (with a trend similar to the predictions based on 2-D simulations). We propose the frequency separation ratios as better observables to determine the fundamental stellar parameters, and to use the large frequency separation and frequencies to extract information about the stellar structure. The effects of diffusion and equation of state on the oscillation frequencies are also studied, but present seismic data do not allow their determination.