This question is lacking sufficient background information for relevant/useful replies. Suffice it to say they can both make great large prints and they can both make awful large prints.
What kind of subject matter?
What kind of shooting style?
On tripod or hand held?
What kind of lens (wide/long/normal)? For that matter, on the H body or on a tech camera body?
etc etc etc
Far better, IMO, to tell us about what you want to do, how you would like to do it (i.e. shooting style) and then ask what the advantages and disadvantages of each system would be for you. There are (IMO) many applications where a digital back will produce a significantly nicer file for printing, and there are plenty of applications where the D800 would be a better tool.
If ALL you care about is print quality at 40x60 then the only meaningful way to answer this question is for YOU to go shoot both in a scenario similar to the one you intend to use it in, and then for YOU to print it (or have it printed) and for YOU to view and compare.