It's a quick question, but there is not a quick answer unless you do an actual test under the conditions that you will use the light.
The capacity of Duracell Coppertops depends on the discharge rate and other factors. You'd also have to know if your light engine is as efficient at 3 volts (2 D cells) as it is at 4.5 volts (3 C cells) and whether it puts out more light at the higher voltage and what the cut-off voltage is.
In general, however, there is 35 percent more energy in 2 D cells than there is in 3 C cells.