In dressage, Hunter Eq. and Pleasure, I always sit the canter.
A hunter O/f or flat class, I two point. It's what's expected where I show and for what I do.
Two pointing, is to get the weight off their back, although unfortunatly, too many riders go straight to a two point in the hunter world, and never learn to sit the canter. They also have a weak core and leg, so it makes it look like their posting, although, let me tell you now, it's NOT proper riding. They're simply weak.
If you have a strong leg, you will be motionless, and not floppy at all. The horses back will be coming up to meet you, but you or your legs never move. Sitting the canter gives you a good center balance, so two pointing will probably be a little easier for you once you get the muscle built for it.
Ask your trainer how he wants you. Knowing how to sit and two point, are essential for me, for what I ride, as I need both. It all depends on what you do.
Sitting, is needed for controling paces, steadying a stride, and being in solid motion. Two pointing is simply to get weight off their back, but it's become a hunter/jumper style. Notice, XC riders two point the whole course, to save their horses from being sat on the whole strenuous time. That's a very proper reason for two pointing. I still have yet to find the purpose of two pointing during hunters. It's just simply a style, not a need.
"You're just as sane as I am."~Luna Lovegood.