as with all "using on bike" pants, one of the main issues is seams, and where they are, how pronounced they are etc. Of course, for a 20 min ride or an hour ride it is one thing, riding all day is another when it comes to chaffing or whatever due to either seams or bunching up.
I'd be a little wary of horse riding stuff just cuz one doesnt have to deal with the constant leg movement while in a saddle (non Brooks)