In fact there may be no answer as the cams are totally different inspite of having similar designations.
A roller cam takes less power to rotate than a flat tappet cam. In a factory engine, that might translate to more power or better mileage.
The benefit of a roller cam in the aftermarket is the aggressive lobe profile that may be used. If you look at a roller cam lobe, it will appear to be almost squarish in appearance rather than coming to a narrow point at maximum lift. A roller gets by with much faster lift acceleration on the profile because the roller lifter with enuf spring pressure will follow the aggressive contour rather than a flat tappet which will tend to be thrown off the cam by the rapid acceleration. Flat tappets are limited as to the profile they can follow with spring pressures that will not flatten the cam lobe.
A 210 roller will have a more squarish lobe than a 210 flat tappet and will near maximum lift much quicker and will stay there much longer. A flat tappet will only be a maximum lift for a brief moment due to the gentle acceleration ramps that much be used to maintain lifter/cam contact.
Because a roller stays at near maximum lift much longer, it will permit much more flow from the same duration cam as a flat tappet.
This means we can get the same flow from a shorter duration roller as we can from a longer duration flat tappet. A benefit for users such as us is that we can use a roller with shorter duration and avoid the problems that a longer flat tappet bring such as loss of low rpm torque, reversion, etc.
Normally, adding lift to a cam does not hurt the low end so, more effective lift from the roller does not hurt us as long as the duration is kept in check.
I run a 210 roller and it pulls harder thruout the entire band than did the 221 deg flat tappet which pulled the hardest up high in the band but gave up some bottom end. The 221 sure sounded good, tho.