I think, horizontal format has both advantages and disadvantages.
Vertical format on desktop is brr, (and You are unable to rotate monitor (well, except Pivot))
Horizontal format on mobile is brr (even when You can rotate mobile, nearly no one use it)
Page (A4 paper) format is better for print...
But because today nearly everyone is addict to mobiles, I would select Vertical format, or combined (responsive) format (but this one format is supported by only few websites, and is possible only when panels has solid border in image, so You can't do panels, that are too big). So in result, vertical format is propably best for mobile adicts...