I'm not sure this can be directly answered... As you already said, horses are different.
I have a really thick loose ring oval mouth, that with a large pony I used it on, there was lots of mouth gaping. While another horse was very content and quiet in it... Different mouths, different preferences on the horses part.
They say thicker bits are milder, because would you rather have your thumb, or a thin wire pressing between your tongue and pallet? Wider range of pressure.
My gelding moves off really nicely in the french link dee made by Korsteel, on a side note.. He doesn't go for thick bits, and I haven't tried anything thinner on him. Best thing to do is borrow, and see what your horse is most content in and take it from there.