Horizontal. I read in the horizontal, my field of vision is greater in the horizontal, and it just looks right to me. But then again, I'm a left-handed dyslexic, So, YMMV.
Below is a 2.5" custom board I had made up in padauk and northern ash with a horizontal grain pattern.
And a custom 2.375" board in bubinga and maple with a vertical grain pattern.
Which do you prefer?
Horizontal. I read in the horizontal, my field of vision is greater in the horizontal, and it just looks right to me. But then again, I'm a left-handed dyslexic, So, YMMV.
Below is a 2.5" custom board I had made up in padauk and northern ash with a horizontal grain pattern.
And a custom 2.375" board in bubinga and maple with a vertical grain pattern.
Which do you prefer?
I've been playing on boards with vertical grain since the 1960s so I would choose the second board you posted though both look attractive from a display standpoint. I think I've been conditioned to expect vertical grain boards given how long I've been using them. I'm also comfortable playing on boards made from non-wood products having no grain at all.
If you think of a paper folding board, the seam is always horizontal. This splitting of the board into "my half, your half" is appropriate and relevant to strategy. I was gifted a wood board that folds along the vertical center, and I find the seam annoying. The grain too should follow the horizon. Horizontal is the more natural view of the world, as is the natural horizon. My vote is horizontal grain.
I like the Horizontal aesthetically and helps to see the lateral movement - not just up and down (rook-ie here) stuff.
ok but in all seriousness do people actually look for that when buying a board? do people go "oh no I lost it's because the wood was going the wrong way"
Either orientation is fine with me, but i do prefer the "vertical" look and feel.
I looked through my boards and was surprised to find no 'default' orientation of wood grain, many horizontal, many vertical, a few mixed. I played all of them, mostly without noticing the direction.
The rules, to play for or against mate, are giving the game a direction towards one another. The starting position at far ends of the board and the pawns moving forward give horizontality and verticality different meanings.
So you might think, if wood grain is strongly accentuating horizontality or verticality, it could surface and resurface as a topic during play, or channel the game, in little ways, subconsciously.
On a lot of boards of mine the grain orientation was not obvious, and maybe Ronbo's argument from #5, 'it doesn't matter, as long as it is not too pronounced', nails it. Quadibloc's conclusion from #7, that light and dark squares should be orthogonally grained and most visibly so, to set apart neighbouring squares most clearly, would result in a sight too blatant and stressful, I believe.
Loubalch's boards from #22 (march 12th, 2019) would have to play totally differently, such outspoken horizontal calm vs. vertical stream 
I´ve got also have boards with vertical, as well as horizontal grain. Both is fine but I find horizontally somewhat more asthetically pleasing.
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Horizontal for me