Plastic vs brass inserts for pex

Does the insert matter for pex? I know the ferrule has to be plastic for pex, but what about the insert?

If it matters, this is for a 1/4" poly/pex/plastic line (whatever is standard with a humidifier - not sure if it's pex) running into a 1/2"x1/4" supply stop. The 1/4" side is a compression fitting.

A brass insert will be thinner than a plastic one, so it will disturb the flow less.