Respective merits of both cameras setups you are considering aside, I think if, you are deriving the majority of your income from stock then both
are far more than the industry requires and neither would offer you a competitive advantage.
The entire selection process is based on small thumbnails and only slight enlargements and delivery is jpeg
Having licensed thousands of images over 3 decades never has one been rejected for technical shortcomings and that includes quite a few pretty weak
scans from 35mm film delivered in the early days of my migration to digital.
The vast majority of your competition in the industry comes from photographers working with 35mm slr or dslr based systems with a rapidly growing number of them
being talented amateurs with good images but not,necessarily, a need to sustain a viable photographic business.
Considering an 5-10 fold increase in your equipment acquisition costs using a medium format back system vs. a 35mm based system the math suggests how many more
sales you have to close simply to break even.
If print sales are a major component, then the DB back choice becomes a more logical option especially if print sizes are regularly on the large side.
As a longtime Canon user, I'd say the D800 is about the best all-rounder available, now, when weighing cost to quality and features