I like how you left the lines visible Smrobs. I just can't let my poor mom see it, she used to work home improvement...she'd want to sand and stain it, LOL.
I like being able to line things up without a level.
Beadboard can give either a cottage look, or a cabin look depending on how wide the lines are. If horses will come in contact with it, don't bother with BB, it's too expensive to have it chewed and rubbed on.
If horses will be able to rub/chew, go with plywood or OSB, but be careful with the OSB it'll leave splinters more easily than plywood.
Smrobs it looks like you have OSB rather than Particle board, if so, good job because PB won't hold up well to wet/dampness at all. OSB can be stronger than plywood according to how it was glued together.