I did some research a while back and posted here:
http://www.dakkadakka.com/dakkaforum/posts/list/544068.page
Essentially it comes down to this.
1.
PSC makes kits with a lot of redundant parts. So you get British commanders with the M4A3 tanks, even though the British didnt use them, the same gun options throughout etc. The only item handled seperately is the Firefly.
2. Given the choice choose M4A1 over A3 as the earlier tanks saw wider action.
3.
PSC provides two bustles for each sprue, enough for one turret of the two available. However turret bustles are interchangable, tanks were jury rigged with any combination of high or low bustle turret hatches, so the bustles on two boxsets will be enough to service both 75mm and 76mm turrets allowing you to field the 76mm with swap out turrets.
4. 105mm tanks used the chassis available, as did the 76mm as both were higher priority to the 75mm for refits an salvage. You tanks dont have to look identical.
Take a mix of the chassis you want.