Straight out of the box the D2000 is the better sounding unit. After burn-in (both require massive amounts of over 400 hours) I still think the D2000 is the better one but they are closer.
Both of these cans need massive mods to sound their best, however with mods the D2000 pulls easily away from the HFI-780. When I say mods I mean they both need a re-cable and in the case of the D2000 the MarkL mods, maybe his wood cups if you want to take it all the way.
I tried thru the generosity of various headfiers three re-cables on the HFI-780. These were the S2 Audio, the ALO Jenna, and the V3 by APureSound. If money is an issue the re-cable by S2 Audio provides the biggest bang for the buck, but IMO the HFI-780 sound best with the V3 by APureSound cable. The re-cables depending on cable length and if you choose dual entry could range from about $150 to $400. The higher cost means a 10 foot long cable, with dual entry, and premium Furutech plug.
In the case of the MarkL mods you could do them for under $25 following the instructions posted by Mark Lawton. Or you could send him your cans for the mods, if you choose this option I would suggest you purchase a set of wood cups (about $225). The wood cups chage although noticeable is not as pronounced as the MarkL internal damping mods, however the wood cups also visually enhance the Denons.
I also found a cable that goes very nicely with the Denons. It is offered by Zynsonic as the XEV cable, a silver plated copper which would set you back about $150. IMO it is the biggest bang for the buck in the case of the Denons.
So in closing the Denon's would be my pick and as a minimum I would get a re-cable by Zynsonic using the XEV cable and I would perform the MarkL mods diy style. Note: If you prefer a warmer presentation Zynsonic offers an all copper cable by Cardas. This way your out of pocket would be under $200.
They sound in their own way (good one)!
