I confess I did not pay much attention to the roof. After years of camping indside/beside a Blazer or a Trooper in the Andes or the Hoggar, ANY hard side camper would be a tremendous step up. There is very little that I want to do when the temperature drops below freezing, so we are only looking for a three season camper.
That said:
-- The original Tiger is evolved from the Four Wheel camper - Dave Rowe was a founder of both companies. The CX/Bengal offer about 1-2" of fiberglas insulation, with the possibility of "cold bridges" in the aluminum frame. You can read tons of discussion of this in the reports of the Turtle Expedition over the years, the prototype Earthroamer, and the blogs of Rob Blackwell and Rick Howe.
-- Mark Guild, the current owner of Provan, now Tiger Adventure Vehicles, is aware that people want more insulation. His answer is the aluminum sandwich construction of the Siberian. I rather doubt that he will do that much more to the CX/Bengal beyond the windows, etc. I do know that he is looking at a better furnace. I suspect that you will see an aluminum sandwich Tiger at about the one ton size in the coming years, but I wouldn't dare to guess when. Right now he cannot keep up with the demand for CX/Bengal Tigers from customers with cash in hand.
The Siberian is not just a bigger Tiger, but a whole new vehicle. Read the specs and options - diesel appliances, cassette toilets, Heike/Dometic windows, LED lighting, hard floors, increased external storage, dedicated electrics cabinet, increased solar capacity. On top of the things that Tiger has long done well or better than most - ample propane (with option to use local small tanks), integrated generator, practical floor plan, ample kitchen working space (look at some of the competition and try to imagine actually preparing a meal in there), etc.
Tigers regularly get trashed on this and other forums for not being "serious" overland vehicles. Indeed at least one Tiger owner gave up on Expo because he was tired of being told that his Tiger "couldn't do" the things that he did everyday because it didn't have a pivot frame, etc. A lot of Tiger owners are simply out there driving around the world and enjoying it in their "toy" overlanders while a lot of the wannabes are killing electrons on the Internet.
We are looking at a Bengal, probably extended cab, but, if I could be sure to have room for camping and recovery junk, would prefer a regular cab for the shorter wheel base. My advice to anyone interested in a Tiger is to send them an e-mail and, better yet, go visit. I don't get paid to shill for Tigers. If Dave Rowe was noted for not being interested in customization, Mark Guild is very different. He is not in the custom coach building business, but he is very interested in talking to current and prospective customers to understand and meet their needs. They might be able to meet yours.