1512/1.5Y vs. 1512/2.5D?

[GriffinRU]

Motor with best efficiency is the perfect balance between copper and core losses. Copper losses associated with current and core losses with voltage. While it is easier to see copper losses (heat) it is not that easy to observe core losses.

So, it would be difficult to see much difference in efficiency among two motors because they run at different voltages and currents. If you compare under the same conditions they both will be not good, but if you put one in sweet spot then another will be in opposite.

Bottom line select you motor based on your gear ratio, battery voltage, ESC used and driving preferences.