Interesting proposition, and we must, of course, make a few assumptions, to wit: A: I will assume that they are flying high enough so that the airframe failure airspeed, which is the same for both airframes, will be approached by both aircraft for 90% or so of the duration of the decent, B: I will assume that the airframe failure airspeed can be exceeded (disasterously) by either aircraft through a combination of power and gravity exceeding the airframe drag at that airspeed.

With only these assumptions, both planes could get to the ground at the same time, but if we assume that the heavier one is prone to exceeding the failure airspeed in a vertical dive with minimum thrust and maximum drag, (and must therefore shallow out the decent angle to keep the airspeed below failure velocity) then the lighter one will be able to safely get there first by diving at the same airspeed and a steeper angle.