Monocrystalline diamond contains a single crystal structure, which has distinct cleavage planes causing the diamond to fracture under stress. This can result in deeper vertical micro scratching on certain materials which is either tolerated or can take longer to polish out.
Polycrystalline diamond has much smaller cleavage planes which causes only surface microfractures. The random structure gives polycrystalline diamond toughness in all directions resulting in less sub-surface damage. Polycrystalline diamond gives faster stock removal and better surface finish especially on harder materials than monocrystalline. The greatest difference in performance is with the finer micron sizes.