Is a vertical or horizontal form progression mental model the best?

Designing a customer facing offer claim web-form, and have arrived at two major directions for the design.

In one the user progresses left to right with the form summary / progress bar displayed across the top.

In the other version the user is going from the top down, with the questions acting as both summary and progress indicators.

Which do you think is the clearer navigation model ?

Jackob's law would suggest the more traditional layout is easier. But the feedback from stakeholders and users has been strongly in favour of the vertical layout.

What do you think might be the downsides of the vertical layout ?