I like french links with round "peanuts" in the middle the peanuts are a lot mider than flat links. A peanut will soften the "nut cracker" effect of a regular snaffle, and also encourages the horse to play and relax into it better.
Mylers have a new concept where the bit can twist from side to side. Some models can collapse slightly but others can't collapse at all. Something people don't often consider with the non-collapsing mylers, is that since they don't have a collasing joint, their action is based far more on the bars of the mouth, much like a standard straight-bar. If the horse is used to a regular jointed-snaffle which acts mostly on the corners of the mouth, using one would be a big change and it would take a bit of getting used to.