This question may never be resolved!!! But it's always interesting...
Interestingly, I just brought a 5 year-old Beneteau
from BVI to FL, and on the shift lever was a a sticker with a clear statement from Yanmar that the prop should never be locked in forward, only in reverse. It didn't discuss the option of neutral.
Regarding which one creates less drag, and with total recognition of the fact that theory and reality are only distantly related in most cases, the reason a prop spins in theory
when an external force (wind/water) is applied parallel to the axis of rotation is that it is trying to minimize resistance. Using deductive reasoning, one is forced to conclude that impairing the ability of the prop to spin must
, therefore, increase resistance (drag). Of course, in the complex reality of an aerodynamic or fluid dynamic system, there may be other factors that modify this relationship, but in my professional opinion (I'm a scientist, not a physicist, but I've had plenty of physics), it's hard to see how you could completely invert these effects. Just my two cents. PS: I let my prop spin freely when under sail. Pete