Bitting can be complicated and it is extremely important. When I was in high school riding hunters and taking dressage lessons everything wore a plain D or eggbut snaffle. Some even used a happy mouth. Pretty much everything we trained in college used a loose ring, hollow mouth snaffle. The little Arab I rode used a mechanical hackamore and that was my first ever experience with that. Turns out what Dr Dunn said was true... Lu is worse with a bit than she is in a hackamore.
I got to the track and learned there a few more bit choices. Ring big, D, loose ring, copper mouth, cage. And now that I'm riding and training my own horse to event I feel as if I've become some what of a bit expert. Twister used to be ridden in the Medieval bit of doom. The fact that I can now ride him in anything less is nothing short of miraculous. I can get through a whole dressage test in a controlled fashion in a french link!
At the same time, however, Twister is a horse that needs to have his bit switched periodically. And sometimes he does his best work, especially jumping, in a mechanical hackamore. I have to be very in tune to how he's acting and feeling to pick the correct bit for each ride to get his best work out of him. Going to shows means I have to have one bridle with the hackamore on it and one with his most recent bit of choice and have a couple other back up bits in the trailer in case I find out during warm up it'd be in our best interest to switch.
Dressage is harder. I have fewer options. So really what I do is school to death in the same bit and hope he remembers that bit means dressage and gives me good work at a show.
Jumping and trail riding..... Is it a trail ride where we just plod along? Or are we going to do hills and jump random logs? (Usually it's the latter....). One time I thought I picked the right head gear and we did a trail ride through some fields and over logs in a hackamore..... needless to say I got R-U-N-O-F-T with and almost fell off. Not my best moment. And in the spring when I'm getting back into riding shape and my legs are all over, the hackamore is the way to go so I don't pop him in the mouth too much on accident.
Twister is also a horse who loves to be heavy on the forehand and occasionally during a canter, yank my arms out. I just got a snaffle gag bit and have been riding flat work in it. He is so light and responsive! I think it might be a good bit for him! But next week who knows.... I might have to switch back again.
Now my friend's mare is so mercurial that somedays she is great in one bit and others you can't ride her in anything, sometimes it's the hackamore (she has like 3 different types!) and sometimes it's a harsh bit and sometimes it's a gentle bit. And no matter what you can't figure her out! But maybe that's just a mare for you. Personally, I think I'll just stick to my geldings and colts!