I would imagine roller on the cam vs roller on the lifter has several advantages and disadvangates. These are the ones I see:
Advantages
1) It is cheaper to make. Roller cam allows the use of the existing head. I'd think roller lifters require additional height.
2) Less mass as Old Mech mentioned. This means you can use lighter springs, so even lower power loss through the cam drive.
Disadvantages
1) Limitations in cam lobe design due to the tip needing to be a radius vs a variable profile.