This is a Matter of Choice..
The Benefits of a 1-Piece Fitting is that it is Convenient and easy to use. The Benefits of a 2-Piece Fitting is that the Ferrule is the only piece that needs to be Replaced when the unit is Worn Out.
For most Applications, they will both provide perfect Connections. For Higher Pressures, it has been Suggested that a 2-Piece with a Double Cone Ferrule will Continue to hold even Though Pressures are above 6,000 psi.