Hello, I'm building a model of a StuG III Ausf.G (mid/late) and I wanted to get some input on what I need. I went to the Missing Lynx forums, but the posting is limited if you're not a mod there. I'm using the Dragon (late 90s kit), Tamiya StuG, and the Revell StuG kits. Which one is better suited for one with Zimmerit?

Alright first things first; reference, I cannot say this enough. How to go about this: You need to know the general month it was built and the manufacturer (IE: Fahrgestellnummer (F.nr.) or chassis number, Alkett et al). This will help with understanding what was and wasn’t used in production. For this find a copy of Achtung Panzer #5, which is devoted to the StuG III/IV. It has clearly illustrated line drawings which shows what went where.
Secondly get a copy of Spielberger/Doyle/Jentz ‘The Sturmgescühtz and its Variants’ book, this will help in clarifying the manufacturer and possibly F.Nr.
Okay with that out of the way, lets discuss (theoretically, of course!) which course of action. Lets say you wanted to do a Oct. 1943 early/mid StuG.III Ausf.G produced by Alkett.
Lets start from the ground up:
Return rollers: Return rollers with rubber are usually found on MIAG or M.A.N. due to their chassis being left over Panzer III Ausf. Ms  that were focused into StuG III production.Alkett used either pressed steel with with spokes (not really common till late 44) or pressed steel with holes. While there is always pictures of some with rubber on the return roller, it’s usually better to model with the steel rollers.
Hull: In January 1943 a directive was sent to all StuG III Production plants for an increase in lower frontal armor plates to be increased from 50mm to 80mm. Due to the backlog of chassis already in existence, a 30mm rolled steel armor plate was bolted. This resulted in MIAG delivering StuG III Ausf.Gs with 50mm hull armor being delivered to the front. M.A.N. examples tended to be welded plates, MIAG bolted and Alkett were made at the required 80mm (although some examples of the bolted type had left the factory).Hulls between January and March 1943; the glacis plates were to be of a single piece with a single hinge instead of the two piece double-hinged hatches, although some 2 piece remained during production.
Front Superstructure: A supplemental 30mm armor plate was bolted over the driver/radio operators compartment.
Gun Mantlet: The two variants of Topfblende or ‘Saukopf’ mantlet which was shaped to deflect shots. One which only had the main-gun housing produced from February 43 and another with the coaxial produced from September 44. There are many cases for mid/late and final using the F and F/8  wedge style welded mantlet.
I could go on, but seriously at this point you see the myriad of production changes just within the same version of just one Ausf.G.
Now to answer the final question, most likely the Revell is the earlier attempt at a StuG by Gunze Sangyo, which was also re-tooled back in Dragon’s early days. The Tamiya is still good for some of the smaller details (hatches and the like) considering it was made a while ago. There are a LOT of after market items to spruce up the Tamiya offering. That said, I’d most likely kit-bash the Dragon/Revell and snatch details off the Tamiya. So get the Achtung Panzer book, figure out which one you’re trying to model and run with that.