it'd be kind of hard to make and also dont forget that you have to turn the whole tank to aim.
It would not be any more difficult to make. Actually, the Su-100 would be easier to make because the T34 chassis is already in game, while the Panzer III chassis of the StuG III would have to be built from the ground up. Food for thought: there were over 10,000 StuG III's built, and "only" 2300 Su-100's.
Both would be good to have, but I'm leaning towards the Su-100 because it offers something we do not have in AH currently. The StuG III would will a void in scenarios.
The real neat thing would be if HTC could add in the StuG and have the different guns available for choice. The long or sort barrel 75mm, or even the StuG 42 version.