Stugs were used exstensivley in the later part of the war by both the Heer and the Waffen SS in place of tanks
Many Panzergrenadier divisions "panzer regiments" were actually composed entirely of stugs and at least 1-2 battalions of the late war Panzer Brigades were stug equipped.
This was done due to their ease of contruction,,(a little known fact was that some of the "armor" on the front of one model was actually formed concrete,I forget which one

So the stugs should be made a priority,,how I'unsure

Marder(s) were used defensivley and usally fired from a concealed ambush position and with infantry support.
Their doctrine was to fire once or twice from a distance and then displace to a new prepared firing postion (I have often wondered if NATO'S Cold War AT doctrine had been contrived from this practice),,thus the high rate of speed and almost a total lack of armor protection. (a .50 cal could penetrate the armor).
They could be used in the DCL as aux. units or as available as core in a defensive orieneted campaign and given a high range value