If you ask me (I don't know why would you do that, though
): while the approach is nearly the opposite in the Zen and the Theravada tradition, the goal is the same. Theravadins tell you almost nothing about the posture to convince you not to fixate over it, and just put the body down. Zen tells you almost everything about the posture to convince you not to fixate over it, and just put the body down.
The first strategy based on the premise that the lack of instruction is what you need to not being concerned, while the second strategy based on the premise that clear instruction is what you need to not being concerned.
Every of the two can backfire: In the first case one can spend his/her whole meditation wondering how he/she should sit, while in the second case one can spend his/her whole meditation worrying about keeping a precise posture. Both problems effectively prevent the stilling of the body, which otherwise happens naturally.
But this is just pure intuition... although it is quite true that in many case the only real difference between the traditions is just the approach, and the essence is the same.
"Just as in the great ocean there is but one taste — the taste of salt — so in this Doctrine and Discipline there is but one taste — the taste of freedom"