What's the vertices and faces count?
I think the barrel shroud/fume extractor on the M1 needs some detail work, but outside of that it's not too bad. Your textures are the weakest point. Most of the pro modellers I work with have that problem. I would like to see what your PZIII and Stug look when you finish and render them. Is that the Porsche turret you used on the K Tiger?
Armor fans know every nut and bolt on every tank and your detail level looks okay, but you seem to have invested a lot of time in your models, so why not go the extra effort to make it the best you can ? Detail is the difference.
Heres a link that you may find interesting.
http://gunpoint-3d.com/model-m3stuart.html