To be fair, I bought my 10.5 primarily for loading and reading magazines on, and to browse the web. For this purpose, the size is excellent, the text is always big enough that you never need to zoom in on a page - sometimes on my iPad Mini I have to zoom on a page if the text in the magazine is not clear enough.
But for about everything else it can be awkwardly large
None of the Samsung stores that I went to - Gateway (Durban) or Canal walk and waterfront (Cape Town) had a display 8.4 that I could check out. They only had the smaller ones in sealed boxes. So....I don't know...maybe fussy me would have complained that the 8.4 would be a bit small, and I would need to zoom some magazine pages like I sometimes do on my iPad mini....I don't know.
But....this 10.5 is an amazing device, make no mistake. I think it's a personal preference wether you prefer the more manageable 8.4 or the large, but lovely display of the 10.5.
Oh...of course, load all your pictures on the 10.5 if you do get one....pictures show up really well on this display.
Oh (mkII)....I prefer to operate a tablet in portrait mode, and I find the buttons on the 10.5 awkward off to the side when I hold it in portrait mode. In the 8.4, they have put the buttons down below, same as in a Note 3 or S4/5 etc which would have been better for me. I don't like landscape mode, except maybe if looking at pictures.
With the 10.5, holding it in portrait mode, and reading a magazine....I am always bumping the damn buttons on the left side, where my left hand is holding the tablet.