I had a hard time knowing what to use at first as well. Leslie’s descriptions were great. You also have to make sure you have the correct size for your horses mouth. Their are mouth measuring sticks for less than $10 or you can use a piece of hay bale tie. You can look up exactly how on the internet. It’s very easy. Too small a bit will pinch and too large has it’s own problems. Also there’s the tongue size and shape of the horses mouth to consider. My off the track thoroughbred has a small mouth and had trouble just closing it around what might have fit most horses his size. I found a curved JP Koresteel egg butt French link that he is very happy with. The curved mouth piece fits the shape of his mouth much better. There was a big difference in how he felt/worked between the regular French link and the curved one.
The previous owner was torturing the poor guy with a jointed kimberwick because he wouldn’t put his head down. He has a very soft mouth and is more slow than go so he did not need such a harsh bit.