Simple question: which of them is more accurate?
I want to build an early version, not the one with the conic turret.
since no one more knowledgeable has answered I'll give you my opinion.
I was in the market for a T-35 kit recently and did a lot searching on the net for reviews. None of the reviews talked about dimensional accuracy so I can't comment on which kit is best in that respect. They may both be very good or bad, I don't know. I have the Zvezda kit and as a kit it looks very good. Nicely moulded and the detailing is sharp. The main turret has a partial interior. The 45 mm gun barrels are multi part and might be better replaced by turned items but otherwise I have no complaints.
Photos of the sprues from the Hobby Boss kit look equally nice and I can't see a big difference in quality between the two manufacturers.
The details of the T-35 changed quite a bit over the production run. Early tanks had large single hatches on the main turret which were replaced by twin hatches on later vehicles. The driver's visor and hatch changed over time. Later tanks had a different exhaust configuration. The number of legs on the turret aerial increased from 6 to 8. Some tanks had hinged trapezoidal hatches on the side skirts to access the suspension. The last tanks had square access hatches. The later tanks had side skirts that stopped short of the drive sprocket to stop mud build up. Of course there was the change from cylindrical to conical turrets. The last tanks had sloped sides to the main turret base. There were also changes to the tool stowage.
The good news is that Hobby Boss seem to have caught most of the variations and the models are generally correct for the period stated on the box.
The final production version should probably have disc idlers but that looks like a not too difficult fix.
I chose to model a particular vehicle, 0200-0 as was lost at the battle for Verba. This tank was extensively photographed so we know what features it exhibited. The Zvezda kit was almost perfect for 0200-0. Cylindrical turret with two hatches, late exhaust, late drivers visor, early driver's hatch and the correct configuration of the side skirts. I believe the tool layout is also correct. The only problem is that the kits lacks the amplified faces for the machine guns which will need to be corrected.
Which kit should you buy? If you are looking for a mid-production T-35 with cylindrical turrets then I might recommend the Zvezda kit. It has the partial turret interior AND was significantly less expensive. I'm very happy with mine. If you have a particular vehicle in mind and it exhibits features different to the Zvezda kit then your best choice will be the Hobby Boss kits. One of their boxings will probably fit the bill. As I said earlier, they look to have done their homework and have captured the period specific changes correctly.
I will be buying conical turret version later this year and am quite confident that the Hobby Boss kit will be a good purchase. I just wish it was a cheap as the Zvezda kit.
In summary, I think the T-35s from both Zvezda and Hobby Boss are good quality kits that appear to be accurate, well moulded and nicely detailed.
I don't think you can go wrong buying either company's kits.
Hope this helps.