Only a number, or for some - the most important number.
If you're regularly printing 24x36 up to 40x60, it is nice to have. The other changes are significant for a pretty small subset of people that usually don't need high resolution - imo. When I was a working photojournalist I would have appreciated those changes, but I would not be wanting anything near a 42 mp camera for daily event work. Way overkill and resource intensive. So in that respect - the A9 would be the preferred solution.
I come from the Canon world where I used 13MP 5D up to 50MP 5DsR.
I am an architectural photographer that has clients regularly print my images up to eight feet wide for their offices.
The res is useful to a point but in truth, if one is shooting for large print the improvement is slight and scarcely visible to the average viewer.
However, I will grant that the hobbyist uninterested in the opinion of the layperson and delights in zooming in to 800%. For them, the more MP the better.
If I am called upon to shoot events I use a 20MP 1 inch Panasonic as it has 15fps, leaf shutter and weighs about 2 lbs. Even then the res is frantic overkill for the final web use.