why such a big differnce?
im sure i cant be the 70 mp more sice the m5 has a better upgrade for the same ammo cost as the 251 and the troops that it trnasports can fire out of it.
i think originally for their rolls, they want SU to be more offensive, give it better assault package and make it able to push with base infantry line, reinforce and provide support with upgrade and 251 give a support defense german infantry push off SU infantry dig in, in a small area and 251 came in to reinforce and help to counter blob anti attack, or urban fight roll with flame. (which OH really need on the map with alot buildings)
another reason is maybe OH infantry have more AT power than SU infantry, M5 use suppression to counter LMG grens as it out performance of cons, LMG have more pen too, and 251 have flame to burn the human wave of SU faster, as OH infantry is usually out numbered by SU.
however since the reinforcement is removed after upgrade, the strategies changes, maybe 251 upgraded should be looked again, as it have less offensive power than M5 upgraded, less usefulness.
another tip for flame 251 though, is flame attack ground don't require vision and can shoot through blockers, and it have quite long range I used a 251 behind a ruin burn decrew a Bofors and destory a 17 pounder and was able to steal the Bofors and turn the game around.