Also a dressage rider. It depends on the level of horse that I'm riding. If I'm riding a youngster that hasn't have it's back as established as a more experienced horse, then I will rise for the majority of the ride until I feel that the horse is ready to take my weight completely.
If I am riding the more experienced horse though, I will rise for about 50% of my ride including my warmup, and the last few minutes of trot work when I allow the horse to go long and long.
As the OP said, in sit trot there is more control. It is far easier to control to tempo, you can feel slight variations in the gait and have more influence over the hindlegs.