Strictly speaking I do not have the answers to your questions (since not yet produced, no one will, I don't think), but I did go through similar evaluation when deciding on an order several months ago.
The following is based on differences gleaned from catalog photos and/or the web representations of same.
MTH: Includes 4 cars, ABS body construction (engine is Injun Joe, followed by Becky Thatcher Baggage, followed by Huck Finn coach, trailed by Tom Sawyer observation. Note that this is from the MTH Website, the labels on the cars were shown differently in the printed catalog.) Link
Lionel: Includes only 3 cars, but is die cast body and vestibule construction (engine is Injun Joe, followed by Huck Finn combo, trailed by Tom Sawyer Observation) Link
For the MTH, since there is no mention of vestibules anywhere in the description (catalog or updated website), I'd have to think the picture is what you probably would get.
I'll leave the prototypical accuracy question to others, but since MTH changed their labeling to match Lionel on the Engine and Observation, I'd suspect at least those 2 are correct.
Personally I chose to order the Lionel and hope maybe someday they might offer an add-on car to bring the set to a similar 4 unit consist as the MTH, but I did not base my decision on that hope. I just like the look of actual metal for a set like this.
-Dave