Having driven 2006 S430 and S500 models back-to-back, I would say that the decision comes down to your desire for the extra torque put out by the S500. Noticeable difference in mid-range acceleration, although the S430 is hardly slow. S500 has higher list of standard features, although most were available as options on the S430.

As they're used vehicles, I would look at their maintenance history as a determining factor.