|1.Besides weight savings? I guess Easier installation and removal in the future without the center bearing and associated bolts that hold it to the undercarriage.
The stock 2 piece is actually silky smooth and engineered very well. I only replaced mine because a u joint was bad and I was told they weren't serviceable on this DS. Also the carrier bearing was worn.
2. I like many others love my Z1 platinum 1 piece.. no abnormal vibrations etc. install was a breeze. Its also clearcoated to avoid rusting/corrosion wear.... oh and avoid anything AMS branded !