Horizontal.

Going to use this large format in my master bath reno. I decided I like the look of vertical tiles to draw the eye up.

I like vertical in the shower. Not sure how easy it would be to transition to horizontal around the tub or how high you intend to tile -- might be nice to keep it vertical there too.

Not a pro, but I vote horizontal if it will work out with the varied levels you need to accommodate without having skinny cuts somewhere that don't line up to the next surface. I hope your installer can give you some sound advice on that. I think going vertical might look like a mismatched attempt to continue the lines of your flooring up the wall.

Thank you for all who responded. Ended up going with vertical. We are incredibly happy with how it turned out. Thanks again for taking the time to comment!

Looks faaaaabulous!!

Abby, looks good. Do you wish your niche was larger?