There’s a ton of scholarship n GUI design. Fitt’s law, etc. No need to speculate on what kinds of thing work.
(I do not know what things work best, just that there are people who do.)
Thank you for your comments.
I think it’s context and user dependent, I agree with @John.
User above all as it’s a human 
Sure you are right, for the need of an intensive data entry you have clearly exposed the shortest way.
If I take my case, I’ve a screen a little bigger than 800x600 and almost don’t use keyboard shortcuts.
If I prefer an horizontal categories’s navigator for my use it’s obviously because a vertical one eats screen width.
With the horizontal I eg can see all colums in People category and having some gramplets on the right and bottom.
As a modest hobbyist i’ve spent around two years to build my family database using almost only People and Family categories during this time, entries inserted one by one following what i discovered (so not an intensive workflow about database).
I have explore other Gramps capabilities only after that (as I had sufficient datas to be exploited).
And comes the times where data entry becomes rare (because as a genealogy research can be infinite I’ve set a limit and have let some jobs for next generations), then we pass in consultation mode.
Here I think horizontal categories’s navigator perhaps has an advantage (eg for Chart and Geographies views), it’s probably the why webbrowsers show their tabs horizontaly.
Now, i think if i had a big big screen and wanted to do an intensive workflow I would reduce the size of application window … that said in absolute terms as I haven’t big big screen.
To be clear also, I dont reject the vertical categories’s navigator and in fact you should do as it is always there (do as if both coexist).
As I discovered gramps code, python and gtk I’ve used one vertical categories’s navigator view to build the horizontal one for ease.
I currently prefer to wait the next Gramps version to integrate it not in a hack way, then both horizontal and vertical will coexist.
It’s always interresting to get user experience.
Best regards.
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