There are two kinds of similar fittings to transition from copper pipe to PEX tubing. Here's an example:
The first is made from copper and looks to be "formed" ie bent / pressed into shape in a mold.
The second is made from brass and looks to me to be machined.
But does it make any difference which of these two types is used? It seems like they will both function identically and be installed the same way.
Note - I'm not concerned about the specific brands, this is just an example.