I'm definatley a poster more of the time. My horse has a very bouncy, pick up trot, thus making it difficult to sit. Although im trying to train him to collect. If it helps my diciplines are Dressage and Hunt seat.
I am riding two horses at the moment, one is my own WB and the other is a TB I am leasing for jumping lessons. I post the trot on both of them most/all the time. Mine because he is young and recovering from rollkur training from his previous owners, and the other because he is extremely out of balance because his rider is heavy handed. Whenever I ride quarter horses I tend to prefer sitting the trot.
I do either or. At the moment most of the horses I ride have issues, so posting is the safest and best thing for me and the horse, as sitting the trot confuses and upsets them more, since they aren't built up and ready for it. One horse in particular thinks that if I sit the trot, I want him to GO, and as he's a huge halter Quarter Horse, with an incredibly bouncy trot, and an inability to canter well due to his extremely large muscling in the wrong areas for riding, going is not a good thing because he is VERY unbalanced, and thus bucks to try and get under himself better. But I am riding one horse that is just a needs to get out ride, bareback more, to help me get back my seat and balance for sitting the trot, and balancing better on a horse.