I've been thinking hard about how I'd answer this one. I had to chose between two cruises that were the same route with one extra day added to one of the sailings. The prices difference was around $800. I ended up choosing the shorter length and cheaper of the two. It was a combination of the price and the date. The shorter cruise dates worked better for me, and I didn't feel like I got enough for me for the extra $800 and inconvenience of the date.
On the flip side I have another cruise where I did choose the longer itinerary at a higher cost, again because the dates worked better.
I think if I was trying to decide I would weigh the dates for travel vs. what I would get out of the cruises. I travel mainly for the itineraries, so even if a cruise is $1000 cheaper but going somewhere I don't care to go, I won't be sailing. I'd rather pay a bit more to go where I want to travel to.
On a purely cost side, in my own small comparisons, adding an extra day to identical itineraries on
DCL seems to add about $300-$400 per person. That seems a reasonable cost for a hotel and food. Will the added day make anything else more expensive, like flights, lawn service etc?
If it all breaks down about the same, I'd always choose the itinerary that I'd rather travel.