The D-40 takes a bit more fuel, and has a few more options- I use it when I have to travel a ways- always use the wing tanks, and take 50% of fuel. Once you shake off the tanks you're ready to rock.
The D-11 on the other hand- I prefer to take up fully loaded. Full tank plus a drop tank. On the average distance, I'll burn through the drop tank climbing to altitude. 100% throttle to good speed at alt, then cut back the RPM's and the throttle to save the gas.
The N....well....if you have to cross the entire map (or you're going buff hunting)- this is your plane. Fully loaded, I think it can stay up for over 2 hours. I'm still undecided on it as a dogfighter because it's WEP *really* helps in a zoom.
Like, outclimb the Spixteen that just came out of a turn zoom. Flop over the top, come down with 8 guns. boom.
One variant I'd love to see is the D-15. Razorback, paddle-prop, and wing racks for bombs/fuel.